“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 Tcell and Bcell count shot back up to normal. Above normal, actually. His gammaglobulin levels peaked. And the weirdest part is, the enzyme I told you aboutthe 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 ADAproducing gene into children with SLIDthis immune problemis either to have a bonemarrow transplant from a twin or via a newfangled genetherapy 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, Kate?”

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. e 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 partsand 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 immunesystem 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 alpineflower 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 unselfconsciousness 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.

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