The process of development—in which a single-celled zygote changes into a multicellular organism—should be, at least in principle, reversible. In the most recent practical demonstration of this prediction, several research groups have shown that small sets of genes, when introduced into somatic cells, can render these cells pluripotent, or able to generate all cell types—in a manner of speaking, make them young again.

Human embryonic stem cells expressing a pluripotency marker.

The stage was set in 2006 and early 2007 with work in the mouse. Most recently, the groups of Shinya Yamanaka at Kyoto University and James Thomson at the University of Wisconsin–Madison have induced pluripotency in human skin cells by expressing a combination of just four genes.

These induced pluripotent stem (iPS) cells share several important features with human embryonic stem (hES) cells, the now classic example of a pluripotent cell type. iPS cells have similar patterns of gene expression to embryonic stem cells, they display key morphological and genetic markers, and most importantly, like stem cells, they can give rise to all three major tissue lineages in vitro and, in grafts into mouse, in vivo. So iPS cells may in the future serve as disease- or person-specific research models but, and in this respect they are distinct from stem cells, without the need for reprogramming by nuclear transfer into human oocytes.

But how similar are iPS cells to hES cells? Are the two reported sets of genes the only ones that can induce pluripotency? Can these cells be created without the use of oncogenes and retroviruses, making future therapeutic use more likely? Can the efficiency of the induction process, extremely low at present, be improved? What will this tell us about the still-mysterious nature of reprogramming and of animal development? Watch for progress in this area in the future.