Csk is a novel cytoplasmic protein-tyrosine kinase that has been shown to inactivate members of the Src family of protein-tyrosine kinases in vitro. To examine the function of Csk in viva, Csk-deficient mouse embryos were generated by gene targeting in embryonic stem cells. These embryos were developmentally arrested at the 10 to 12 somite stage and exhibited growth retardation and necrosis in the neural tissues. The kinase activity of p00’““, p59’rn, and p53/5@’in these embryos was greatly enhanced as an apparent consequence of enhanced specific activity. The increase in klnase activity was associated with an increase in tyrosine phosphoryiation of several proteins, especially those around 85 and 120 kd. Thus, these results suggest that Csk indeed acts as an indispensable negative regulator of Src family kinases in vivo. introduction

Increasing evidence has shown that protein-tyrosine kinases play essential roles in the signal transduction pathways that regulate cell proliferation, differentiation, and function. The non-receptor protein-tyrosine kinases of the Src family are anchored at the inner surface of the plasma membrane through their myristoylated N-terminus and have been proposed to associate with cell surface receptors; the binding of ligand to receptor causes rapid activation of Src family kinases and activation of intracellular signal transduction pathways (Eiseman and Bolen, 1990). Nine Src family kinases have been identified, and several show tissue-specific expression. In particular, the expression of~ 56~~ is confined to cells of the lymphoid lineage and has been shown to associate with CD4 and CD8 T cell surface antigens, thereby transducing independent signals during the process of T cell activation (Veillette et al., 1988). The association of other Src family members (ie, the c-src, c-yes, fyn, Iyn, and blkgene products) with hematopoietic cell surface receptors has also been reported (Burkhardt et al., 1991; Sugie et al., 1991; Yama-