The SH2 domain of Chk binds to numerous pTyr-containing receptor kinases, such as ErbB2 (Zrihan-Licht et al., 1997; Zrihan-Licht et al., 1998; Bougeret et al., 2001; Fu et al., 2006), c-Kit (Price et al., 1997), TrkA (Yamashita et al., 1999), and RAFTK (McShan et al., 2002) in numerous cancer cell types and regulate their proliferation, while the Csk SH2 domain does not bind to these receptors. This evidence concerns the gene CSK and cancer.