Kate started an old game that she and Tom used to play on their outings. “What’s that tree over there?” she asked.

  Tom did not even glance behind him. “Ponderosa pine.”

  “I knew that,” said Kate.

  “Then give me a hard one.”

  She scooped up some of the sandy, pebbly soil they were sitting on. “What do you call this stuff?”

  “Dirt,” said Tom. He was building a gargantuan sandwich.

  “Come on now, Balboa,” she said, “be pacific.” It was an old, dumb joke of theirs.

  Tom used his free hand to lift a bit of the soil. “This is called gruss,” he said. “It’s just a worn-down version of the granite that makes up these hills.”

  “What wears it down?” Kate rarely grew tired of this game. Most of what she knew about nature came from talking to Tom.

  “The granite?” he said and took a big bite of his sandwich. “Ice expanding and contracting. The plant roots. The acid from fungi hyphae in those lichens. Given time, living things will chew the crap out of any mountain. Then the organic stuff decomposes, we get critters burrowing in it and enriching it further when they decompose, and voilà!…dirt.” He took another bite.

  Kate ran her hand across the sparse grass and low weeds where Joshua was crawling. “And what’s this?”

  “Blanketflower,” said Tom around his sandwich. “That jagged thing you don’t want Josh to tumble into is prickly gilia. Those sharp little jobbies are mistletoe stems and the involucral bracts of gumweed. That scabby stuff on the rock is crustose lichen. That other stuff we have a sort of technical name for—”

  “Which is?”

  “Grass,” said Tom and took another big bite.

  Kate sighed and leaned back on the blanket, feeling the fierce sun on her skin. The breeze stirred the high grass, cooled her, and then died, allowing the sun to fill her senses again. Kate knew that she should not feel so totally content with her ex-husband and a sick child, but at the moment everything was perfect.

  She opened one eye and looked at Tom. His blond hair, always thin on top, had thinned a bit more, allowing his eternal sunburn to creep higher on his forehead, but other than that detail he looked just like the overgrown boy she had met and fallen in love with fifteen years ago. He was still fit and almost obscenely healthy looking, his forearms sculpted in the muscled symmetry only rock-climbers could develop. His face, pink-skinned and unlined, fell into the pleasant, unselfconscious smile lines of someone pleased with not only where he was, but who he was. Tom greeted every day as if he had just arrived on planet earth, fresh and rested, and had too much to do and see that day to cram into twenty-four hours. On the other hand, Kate admitted, he never seemed rushed or in a hurry. Living with him had been like hiking up a mountain with him—steady, unforced, taking time to see and know the names of all the flowers, but never turning back short of the goal.

  It was just, Kate realized now, that they had never agreed on a goal.

  Joshua’s arms slipped and he went facedown into the grass. Tom lifted him and set him down on a softer surface. The baby sat for a minute, balancing, and then teetered sideways. He began moving again, pausing only to check the taste of the soil he was creeping across. He did not like it.

  Tom watched him. “Shouldn’t this guy be walking pretty soon?”

  Kate pulled a strand of grass and chewed on it. “He could be already if his development had been normal. As it is, he’s still several months behind. We’ll be lucky if he walks by the time he’s fourteen months old.”

  Tom poured them each coffee from the Thermos. “OK, why don’t you tell me what the tests showed. I’ve been waiting all week.”

  Kate held the plastic cup under her chin so she could inhale the rich scent of the coffee. “The results were crazy,” she said.

  Tom arched an eyebrow. “You mean screwed up?”

  “No, the data is accurate. It’s just that the results are crazy.”

  “Explain.” He leaned back on one elbow. His blue eyes were clear and attentive.

  Kate concentrated on keeping it in layman’s terms. For all of Tom’s interest in the natural world, medicine—at least above the first-aid level—had never been something he spent time trying to understand.

  “You remember,” said Kate, “that I told you about the cyclical nature of Josh’s immune deficiency problem and how it seemed to relate to the transfusions he was receiving?”

  “Yeah. But you said that that shouldn’t be the case. Wasn’t it bone marrow, not blood, that would help the kid’s immune system?”

  “Right. Well, I’ve seen the results of our last test, and there’s no doubt that the blood transfusion has brought about an amazing recovery in his immune system. Within an hour of the injection of whole blood, Joshua’s WBCs were back to normal—”

  “What’s WBC?” asked Tom. He was watching the child crawl while he listened.

  “White blood cells.” Kate took another sip of coffee. “More than that, his T-cell and B-cell count shot back up to normal. Above normal, actually. His gammaglobulin levels peaked. And the weirdest part is, the enzyme I told you about—the one that’s totally lacking in his system?”

  “Adenosine something,” said Tom.

  “Adenosine deaminase. Right. Well, his ADA count had returned to normal within an hour of the transfusion.”

  Tom frowned. “But that’s good…isn’t it?”

  “It’s great,” said Kate, trying not to get emotional, “but it’s impossible.”

  “Why?”

  Kate lifted a twig and drew a circle in the pebbly soil as if she could explain it all with diagrams. “ADA deficiency is a genetic failure,” she said. “The gene for ADA is on chromosome 20. We know how it works, although we’re not sure why it’s so important. I mean, the reason for toxicity of adenosine metabolites isn’t totally resolved, but we know it has something to do with inhibition of nucleotide reductase by deoxyadenosine triphosphate—”

  “Whoa!” said Tom and held up one hand. “Back to why it’s impossible.”

  “Sorry.” Kate rubbed out the circle and squiggles in the soil. “It’s a genetic defect, Tom. The gene is either there or it isn’t. We can look at one, of Joshua’s red blood cells and see whether ADA is being produced or not.”

  “Is it?”

  Kate chewed her lip. “No. I mean, no, it’s not being produced naturally. But, after a transfusion, his immune system suddenly comes up to snuff and he produces ADA like it’s going out of style.”

  Tom nodded. “But you don’t see how he’s getting this new ability to create the stuff. I mean, you can’t borrow a gene from somebody’s blood, right?”

  “Exactly. The only way we can get that ADA-producing gene into children with SCID—this immune problem—is either to have a bone-marrow transplant from a twin or via a newfangled gene-therapy program that’s just been developed where you splice the human gene into the patient by using a virus—”

  Tom blinked. “A virus? Wouldn’t that make the patient sicker?”

  Kate shook her head. “Viruses don’t have to be harmful. In fact, most are harmless. And for the gene therapy, retroviruses are perfect.”

  Tom whistled. “Retroviruses. We’re in AIDS territory here, aren’t we, Kat?”

  Still almost lost in thought, Kate nodded. “That’s what makes this kind of gene therapy so interesting. The HIV retrovirus really shook us up because it’s so lethal, but the cloned retroviruses the gene therapists use are harmless. Retroviruses don’t even screw up the cell they’re invading. They just break into the cell, insert their own genetic programming there, and let the cell go about its business.”

  Tom sat up and poured more coffee for each of them. Joshua had crawled in a full circle and now returned to play with Tom’s shoelaces. He managed to tug one until it was untied. Tom grinned and untied the other one for him. “So you’re saying that this gene therapy could save Joshua’s life by getting some harmless retrovirus in there to fake out the
cells into producing ADA.”

  “We could,” said Kate, sipping at her coffee, gaze unfocused, “but we don’t have to. Somehow Josh uses the blood we inject to do the same thing. Somehow his body breaks down the genetic structure of that blood, finds the cellular building blocks it needs to overcome his own body’s immune deficiency, and trucks it around his system within an hour.”

  “How?” said Tom. He ruffled the dark fuzz on Joshua’s head.

  “We have no idea. Oh, we’ve found what Alan calls a ‘shadow organ’—a thickening of the stomach wall that might be the site where blood is absorbed and deconstructed for its constituent genetic parts—and my guess is that Joshua’s body carries its own neutral virus to disseminate the new genetic information around his body, but the actual mechanism is unknown to us.”

  Tom lifted the child high. Joshua’s face showed a second of alarm and then filled with pure pleasure. Tom swung him around in an airplane and set him back on the grass. “Kate,” he said, “are you saying that your child is a mutant of some sort?”

  Kate paused in the act of getting out jars of the baby’s applesauce and strained carrots. “Yes,” she said at last, “that’s exactly what I seem to be saying.”

  Tom leaned over and touched her wrist. “But if his mutation allows his body to overcome its own whatchamacallit deficiency, beat the immune-system problem, then it may hold the cure for—”

  “For AIDS,” finished Kate, her voice raw. “And for cancer. And for God knows how many other scourges we’ve had to suffer throughout the whole history of humankind.”

  “Jesus Christ,” whispered Tom, looking at Joshua in a strange way.

  “Yes,” said Kate, and opened the jar of applesauce to feed her baby.

  Kate had not planned for it to happen that night. She and Tom had come close to making love several times in the years after their divorce, but each time one or the other or both of them had thought better of it. In the past couple of years, their new relationship as friends had seemed too important to jeopardize by resuming a sexual relationship which each knew to be an emotional dead end.

  But this Saturday night had been different. It was just the two of them and Joshua in the house; Julie was off on an alpine-flower collecting tour somewhere near Lake City. They had barbecued chicken out on the patio, moved around to the terrace on the west side to watch the sun set north of Long’s Peak, and sat drinking wine and talking until the sky was strewn with stars. It had seemed natural when Tom had finally set down her wineglass, taken her hand, and led her into the bedroom they had shared until six years ago.

  Their lovemaking had been urgent but gentle, heightened by the unself-consciousness that only an intimate familiarity with the other person’s body can bring, and tinged a bit with sadness as they lay in each other’s arms afterward.

  “Would it be better if I left?” Tom had whispered sometime after midnight.

  Kate had stirred against him, set her hand on his chest. Tom no longer lived near Boulder, but in a renovated cabin near Rollins Pass, an hour’s drive away up Boulder Canyon and south along the Peak to Peak Highway. The thought of him driving it so late at night made her heart sad. “No,” she whispered, “it’s all right. Julie won’t be back until late tomorrow evening. I have some bagels in the freezer and Toby’s going to deliver the Times when he comes up to work on the satellite dish tomorrow morning.”

  Tom had touched her head gently. One of the few rituals of their marriage that each had enjoyed had been a slow Sunday morning with bagels, coffee, and The New York Times.

  He kissed her on the lips. “Thanks, Kat. Sleep well.”

  “Same to you,” Kate had mumbled, already sliding into a contented sleep.

  She awoke suddenly and completely. The alarm clock dial read 3:48 A.M. Kate was sure that she had heard something. For a relieved second she remembered that Tom had stayed and assumed she had heard him padding around in the bathroom, but when she sat up in bed she realized that he also was sitting up in bed, listening. There came a second sound from down the hall.

  Tom put his hand on her mouth and whispered “Shhh” in her ear. Another soft sound came from the dining room.

  Tom leaned close again and whispered, “Would Julie be up here if she came back early?”

  Kate shook her head. She could hardly hear her own answering whisper for the pounding of her heart. “Her rooms are downstairs. She never comes up at night.”

  She could see Tom’s head silhouetted against the faint starlight from the patio. A chair in the dining room was bumped softly. Kate heard the loose floorboard squeak at the far end of the hall.

  Tom slid noiselessly out of bed but leaned close to whisper in her ear again. “Is the twelve-gauge where I left it?”

  For a second the question did not register, but then Kate remembered the argument about the shotgun that Tom had insisted she keep if she was going to live alone out here. They had compromised when he had put it far back in the unused corner of her closet. She’d meant to get rid of the thing, but after a while she had just forgotten about it. She nodded.

  “Did you load it the way I told you to?” he whispered.

  There was another squeak in the hallway. Heart pounding, Kate shook her head.

  “Shit,” whispered Tom. He was crouched by the side of the bed. His lips touched her ear again. “Is the box of shells still on the top shelf?”

  “I think so,” whispered Kate, Her mouth was terribly dry. She strained to hear any sound. Suddenly a door creaked and she swung herself out of bed. “The baby’s room!” she said aloud.

  Tom moved incredibly fast. The closet door slid open with a bang that almost made Kate scream, he flicked the light on, came out with the pump shotgun and a yellow box of shells, stopped Kate from running down the hall by pressing his hand flat against her chest before she could get out the bedroom door, and shouted, “We have a gun!” as he fed three shells into the magazine.

  They both heard Joshua’s door slamming open.

  Tom was out the door and down the hall in an instant, slapping light switches as he ran. Kate was a half-second behind him. She froze as she stumbled through the doorway into Joshua’s room.

  A tall man dressed in black was bending over Joshua’s crib. In the second before Tom hit the light switch in the room, Kate saw only the black shape looming over her baby, the man’s thin face illuminated by the night-light next to the crib. His fingers were long, gloved, and reaching for her son.

  Tom snapped on the light and crouched with the shotgun pointed. “Don’t move a fucking muscle!” he shouted, his voice strong and in command. He was still naked; his body looked tanned and powerful to Kate rather than made vulnerable by the nudity.

  The intruder was wearing some sort of black balaclava, but his face was uncovered. He had a wide gash of a mouth, long nose, heavy brows, and eyes that looked like black pits to Kate. This is a nightmare, she thought through the frenzied pounding of her heart.

  Kate was sure the intruder would use the baby as a shield, but the man stared at Tom from the black pits of his eyes and then lifted his spidery hands and stepped away from the crib. Tom shifted to his left to keep the field of fire away from the baby, and Kate slid along the wall behind him.

  “Freeze,” Tom commanded and pumped a shell into the chamber.

  The man in black seemed to nod, and then everything happened very fast.

  Kate had seen Tom’s reflexes in action before—catching a rafter who had gone over the side in heavy rapids, going on belay to arrest her fall while teaching her to rock-climb, once leaping to stop Kate from a nasty collision with a rock while glissading down a snowfield—but the man in black moved so quickly that even Tom did not have time to react. One second the intruder was ten feet across the room, arms extended, and the next instant the black form was hurling across the carpet in a tuck and roll, then exploding upright under the shotgun, hands going for Tom’s throat.

  Tom was the strongest man Kate had ever known, but the intruder lifte
d him like a child and tossed him across the room. A mobile came crashing down, Tom slammed into the framed N. C. Wyeth print on the far wall, and then he bounced and rolled while the man in black leaped after him. Somehow Tom had held on to the shotgun.

  “Down, Kate!”

  She had run toward the crib but dropped to the floor on Tom’s command. Kate saw a glimmer against the black gloves and realized that the man had a blade in his hand and that the hand was rising above where Tom was sprawled…

  Kate’s scream and the shotgun blast were simultaneous.

  The intruder’s downward leap suddenly reversed itself as if a film were being run backward; he flew up and back, slammed against the wall where Kate had been standing a moment earlier, and slid to the floor. He left a trail of blood and black wool on the ducks-and-airplanes wallpaper.

  Kate ran to the crib and lifted Joshua out. The baby was screaming, face red with terror at the sudden sounds, but he looked unharmed.

  Tom got to his feet, his left arm obviously injured, and approached the fallen man carefully. The knife the man had held now lay on the carpet. Kate had never seen anything so short and deadly. It had no handle, no hilt, merely a flat knob that she guessed would fit in the palm of his hand. Both sides of the blade were obviously razor sharp.

  “Careful!” began Kate as Tom turned the slumped form over with his foot. She caught her breath. The shotgun blast had tom a foot-wide hole in the man’s chest and upper abdomen, while some of the pellets had struck his throat and face. There was much blood. Kate stared for a long second before her medical training took over. She kissed Joshua, set him back in his crib, and crouched next to the man. Blood wicked onto the hem of her silk nightgown and she brusquely flipped it out of the way, tore the remnants of the man’s black sweater open, and felt for a pulse at the base of his neck. There was none. The intruder’s eyes were slightly open but the pupils had rolled up until only the whites showed.

  “Call 911 and tell them to have an ambulance dispatched from Community,” she said. She arched the man’s head back and opened his mouth to clear it of blood and tissue fragments.