Primary structure of Abl kinases. Only isoform 1b of c-Abl and Arg in mice is shown. Both have a “Cap” region at the extreme N-terminus (Cap, gray), followed by the Src homology-3 domain (SH3, orange), Src homology-2 domain (SH2, blue-green), and catalytic domain (kinase, purple). However, at the extreme N-terminus, v-Abl contains a viral “Gag” sequence (Gag, green) by replacing the “Cap” and the SH3, and Bcr-Abl contains the N-terminal portion of the Bcr protein (Bcr, magenta) by replacing the “Cap,” which were generated by uptake into a retrovirus and chromosomal translocation, respectively. In the C-terminal half, Abl has four P-x-x-P motifs and Arg has three in the proline-rich region (PR, pink). At the extreme C-terminus, there are actin-binding domains in Abl and Arg (AB, beige). In addition, Abl has a DNA-binding region (DB, light blue), while Arg has a microtubule-binding domain (MT, blue). Amino acid residues of each gene in mice are numbered except for the human P210 Bcr-Abl gene [42]. The primary structure of c-Src is shown for comparison [43].