PBX1 and leukemia: Specifically, the relative expression of messages encoding proteins known to either associate directly with Meis1 (Pbx1, Hoxa9 and Hoxb3) [33] or hox members known to be expressed in hematopoietic stem cells/progenitors during normal development (Hoxc4) [42] or ectopically expressed upon undergoing translocations in leukemias (Hoxd3) [43], from multiple hox clusters were interrogated.