Live long and prosper

In this issue of Molecular Pharmacology, Meyn et al. (p. 1320) provide the results of the first comprehensive investigation of the expression pattern of the Src family of nonreceptor tyrosine kinases (SFK) in mouse embryonic stem (ES) cells. They found that self-renewing ES cells express seven of the eight mammalian SFK members and that some undergo distinct expression changes during early differentiation events. One of the most dramatic changes was in Hck transcript levels, which decreased almost 30-fold during the first 3 days of embryoid body formation, a culture system model of early embryogenesis and differentiation. Other SFKs, such as Fyn and Src, remain present and active as ES cells differentiate. Of particular interest was the observation that ES cell self-renewal or differentiation can be manipulated through the selective pharmacological inhibition of SFK members. This information should help in the expanding efforts to exploit ES cells for basic and clinical purposes.