The proto-oncogene c-Myc has been implicated in the genesis of many different types of tumors, but its role in normal physiology remains unclear. In December 13 Nature, Andreas Trumpp and colleagues from University of California at San Francisco show that in mammals c-Myc controls the decision to divide or not to divide and thereby functions as a crucial mediator of signals that determine organ and body size.

Trumpp et al. generated an allelic series of mice in which c-Myc expression was incrementally reduced to zero and observed that fibroblasts from these mice show reduced proliferation, and after complete loss of c-Myc function they exit the cell cycle. In addition, in vivo, reduction of c-Myc levels results in reduced body mass owing to multiorgan hypoplasia and not to hypotrophy (Nature 2001, 414:768-773).

This suggests that c-Myc mediated responses controls mammalian body size and that these mechanisms...