Abstract
It is rare that a scientific subject causes controversy not only among scientists but also among politicians and the public. Stem-cell biology is such a subject. The media reports new claims as soon as (or before) they appear in the scientific literature or are presented at scientific meetings. Talk-show hosts present their views, and both Congress and the executive branch follow with policy initiatives. But in its present state, there is little certitude in the science of stem cells, and the hard facts are emerging slowly. It is important for the public and policy makers to understand the fundamentals of . . .