Bcr/Abl protein expression is constitutively increased in P210/STAT5A^–/– CML. (A) Leukemia cells from P210/STAT5A^–/– or P210/STAT5A^+/+ mice with CML or CML/ALL were analyzed by anti-Abl or antiphosphotyrosine immunoblot. Ba/F3 cells expressing P210 (+) or not (–) are shown at far right as controls. Actin serves as a loading control (bottom panel). (B) Leukemia cells were analyzed by western immunoblot using an antibody specific for STAT5A (5A), or an antibody recognizing both STAT5A/B. Controls for STAT5A/5B and their migration positions are shown at far right. Note that the STAT5A-specific antibody (top panel) cross-reacts slightly with the slower migrating form of STAT5B.