Thomson, who in 1998 became the first scientist to extract human stem cells from embryos, says his group began seeking these factors four years ago, but chose to work with human cells. As of last spring, he says, his group, led by lab member Junying Yu, had pared an initial list of 100-plus genes to 14.

Then came Yamanaka, who Thomson says beat him to the punch because mouse cells grow much faster than human cells do, allowing more rapid experimentation. "We thought, 'oh no, it's already been done; we've been beaten,'" he recalls.

http://www.sciam.com/article.cfm?id=human-stem-cells-no-embryo&page=2