Updated project metadata. The two major isoforms of the oncogenic Bcr-Abl tyrosine kinase, p210 and p185, are expressed upon the Philadelphia chromosome translocation. p210 is the hallmark of chronic myelogenous leukemia, whereas p185 occurs in the majority of B-cell acute lymphoblastic leukemia. Differences in protein-protein interactions and activated signaling pathways that may be associated with the different diseases driven by p210 and p185 are unknown. We have performed a quantitative comparative proteomics study of p210 and p185 and found strong differences in the interactome and phosphoproteome. Whereas the AP2 endocytosis complex interacts preferentially with p185, the phosphatase Sts1 is enriched with p210. Stronger activation of STAT5 and ERK1/2 is observed with p210, whereas Lyn is activated by p185. These results were validated in human p210 and p185-positive cell lines. Our findings contribute to a more coherent understanding of Bcr-Abl signaling, mechanisms of oncogenic transformation, resulting disease pathobiology and responses to kinase inhibitors.