Domain stuctures of Abl, Arg, fly Abl, and Bcr-Abl. At least two alternatively-spliced amino termini have been reported for Abl and Arg.116 Only the myristoylated (myr) form (type IV for Abl, type 1B for Arg) is shown here. Abl (mouse): Following the N-terminal exon, the indicated features are Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase (kinase) domains, 3 Pro-X-X-Pro motifs (pink stripes), 3 nuclear localization sequences (green stripes), a DNA binding domain, a globular- (G-) actin-binding domain, an F-actin binding domain (F-actin), and a nuclear export motif (purple stripe). Arg (mouse): Following the N-terminal exon, the indicated features are Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase (kinase) domains, 3 Pro-X-X-Pro motifs (pink stripes), an internal talin-like F-actin binding domain (light green F-actin), a microtubule-binding domain (MT) and a second calponin-homology F-actin-binding domain (light blue F-actin). Abl (fly): Based on sequence comparisons with mouse Abl and Arg, fly Abl is likely to be myristoylated at the N-terminus, but this has not been directly demonstrated. Like Abl and Arg, fly Abl contains the SH3, SH2, and kinase domains arranged in tandem. Sequence comparisons suggest that the fly Abl C-terminus contains an F-actin-binding domain, although this domain has not yet been shown to bind F-actin. Bcr-Abl (human): The N-terminal portion of Bcr is fused to Abl just N-terminal to the Abl SH3 domain. The Bcr moiety contains an N-terminal coiled-coil (CC) domain that mediates tetramerization of the fusion protein. Bcr also contains a Dbl domain with guanine nucleotide exchange activity for the Rho family GTPases Rho, Rac, and Cdc42. Different break points in Bcr can yield 210 kDa fusion proteins with different amino acid lengths.