The intestine is a complicated organ, doing different things at once. It's absorbing some things (such as nutrients) and excreting others, all in a "highly orchestrated" fashion, Capecchi said. The indication is that stem cells at various positions, slightly different from each other, give rise to the needed cells quickly.

He said a group of researchers in the Netherlands, using similar technology, found a different marker than Bmi1 to label stem cells in the intestine. The organ has been well studied, but until recently there was no ability to look at a genetic level since there were no molecular markers. Some have believed the stem cells are located in a crypt at the bottom of the intestine, while others believed they are in a crypt at the top. The U. and Netherlands researchers made opposite findings.

Some researchers have said stem cells proliferate and replicate, then the progenitor cells start migrating up the villi, changing into different kinds of cells to create the various properties of the intestine. Others have argued they march down from a crypt at the top. The U. researchers found 95 percent of the time the stem cells are at the top, while the Dutch researchers found 95 percent at the bottom.

"I think we're both right. I think we're labeling the stem cells at different phases of what they're doing," said Capecchi.

Each time a stem cell replicates, it makes two daughter cells and "sometimes they're not going to be perfect, leading to mutations" in DNA, he said. More replication means more potential for mistakes. So while one of the two copies replicates over and over then dies, the other sits quietly, a kind of backup to ensure the genome doesn't accumulate a lot of mutations, he said.

"I think we're seeing two populations of stem cells. We're putting labels in the cells that are setting aside and he's labeling those that are replicating," indicated by the fact that in the Netherlands research, the color label is getting into its progeny faster than it's happening in the U. research. "Ours are the more quiescent."

That also indicates to Capecchi that the cell's location controls what's happening and whether one is waiting or replicating. The cells, he says, seem to migrate back and forth.