The Old Passengers

The gel image was supposed to show one band.

It showed three.

Soren counted them again. Still three. He wrote the number in his notebook, then looked up at Maya, who was already leaning over his shoulder.

"Your sister is going to think we broke something," she said.

"We didn't touch it. The gel was already running when she left."

"So those bands were already there."

"Yes."

Maya pulled up the stool and sat down.

The printout next to the gel reader had a label at the top: ERV3-1 expression, placental tissue. Below that, a lot of numbers Soren didn't recognize. But the image of the gel was clear. Three bands where there should be one.

"ERV," Maya said. She tapped the label. "What's that?"

"Endogenous retrovirus."

Maya looked at him.

"I looked it up last month," Soren said. "There was something in a paper my sister left out. I only understood part of it."

"What part?"

He turned to a page near the back of his notebook. He had written it down because the inside of his head had not felt large enough to hold it. He read it out loud, in the flat careful voice he used when he was reading something he didn't want to get wrong.

"Viruses that infected our ancestors millions of years ago. Not recent. Ancient. They got into the germ cells, the eggs and sperm, and when those cells became people, the virus came along. And those people had children, and the virus came along again. And again. For millions of years. Until now."

The lab was very quiet.

"They're still in us," Maya said.

"About eight percent of our genome. Not doing anything for most of them. Just riding along."

Maya was quiet for a moment. Then she said: "Most of them."

Soren looked at the three bands.

"My sister studies the ones that do something," he said. "This one. ERV3-1. It's active in the placenta."

"What does it do there?"

"Something about the cells fusing together. The outer layer of the placenta is made of cells that have merged into one big connected sheet, and this protein, the one the old virus left behind, that's what tells them to fuse. Without it, the placenta doesn't form right."

Maya stared at the gel. "So the thing that makes it possible to be born."

Soren had not thought about it quite that way before. He wrote it in his notebook anyway, because it was the kind of sentence that needed somewhere to live.

Maya got up and walked to the window. She pressed one palm flat against the glass and looked out at the parking lot, which was mostly empty because it was Saturday.

"Does anyone know why they stayed?" she asked. "Like, was it on purpose?"

"Nobody knows. Some of them probably just didn't get switched off. Random. Some of them might have gotten kept because they did something useful. Maybe both. There's no way to know yet, for most of them."

"Most of them," she said again.

He recognized this. Maya keeping a door open. She had a list in her head of things that didn't make sense yet, and she was adding to it right now, and she was not the slightest bit bothered by that.

Soren looked back at the three bands.

The protocol sheet said there should be one band for ERV3-1 under the experimental condition his sister was testing. One band. He read the protocol slowly. The sample was from a cell line that had been modified to silence a specific transcription factor, which Soren only partially understood, but the point was: under this condition, this gene should not be active. One band. The control band only.

Three bands.

He looked at the label on the sample tube. He looked at the protocol. He looked again at the label.

The sample was labeled from a different cell line than the one in the protocol.

His sister had used the wrong cells.

Soren did not panic. He got out a fresh sheet of paper, not from his notebook, and wrote down exactly what he saw: the label, the protocol, the expected result, the actual result. He wrote it in the clearest handwriting he had. He put it next to the printout with the gel image.

"She mixed up the samples," he told Maya.

Maya came back to the bench. She read his note. She looked at the tubes.

"So the three bands aren't wrong," she said. "They're the right answer to a different question."

"Yes. And we don't know what the question was, because we don't know what this cell line does differently."

Maya looked at the three bands for a long time. "So there's a gene that ancient viruses left in us, that helps babies get born, and your sister was trying to turn it off to see what happened, and she accidentally tested something else, and the something else also shows the gene turning on, just differently."

"We don't know why it's on in the wrong cells. We don't know if it's wrong. We just know it wasn't the plan."

"But it's real."

"It's real."

The door at the end of the lab opened. Soren's sister came in carrying two coffees and looking at her phone. She was already talking before she reached them.

"Sorry, there was a line, don't touch the, wait, are you, did you," and then she saw the note on the bench.

She picked it up. She read it. She looked at the gel image. She looked at the tube.

She set down both coffees very slowly.

Soren watched her face change.

He did not know the name for the expression she was making. It was not the face of someone who had made a mistake. It was the face of someone standing at the edge of something they had not planned to find.

She picked up the sample tube and held it up to the light, turning it, reading the label one more time.