In fact, the mammalian PBX1 protein was first identified in a chromosomal translocation [t(1;19) (q23;p13.3)] in pre-B cell acute lymphoblastic leukemia (ALL) that resulted in the expression of an oncogenic E2A-PBX1 fusion protein (Carroll et al., 1984; Williams et al., 1984; Kamps et al., 1990, 1991). This evidence concerns the gene PBX1 and acute lymphoblastic leukemia.