Cas contains an SH3 domain (SH3), proline-rich sequences (Pro), a substrate region containing 15 YXXP repeats, a serine-rich region (Ser), and a C-terminal Src binding site. The Src binding site contains a Pro-rich sequence that binds to the SH3 domain of Src, and a phosphorylated Tyr (Y668) that binds to the SH2 domain of Src. The positions of the four YQXP motifs and nine YDXP motifs are indicated.