Recent studies have demonstrated that the E2A gene encoding helix-loop-helix proteins E12/E47 is fused to a novel homeobox gene PBX1, as a result of the t (1;19) translocation observed in pre-B cell acute lymphoblastic leukemias. Molecular diagnostic techniques, including PCR, of the t (1;19) are now possible and provide an useful tool in conjunction with cytogenetics to identify involvement of E2A and PBX1 genes in acute lymphoblastic leukemia.