Specifically, FISH using both TCF3 split signal probes in conjunction with PAX5 locus-specific deletion probes suggests that PAX5 is a secondary event in the oncogenesis of TCF3-PBX1-positive B-ALL, and may be associated with clonal evolution of the malignancy [10]. The gene discussed is PBX1; the disease is precursor B-cell acute lymphoblastic leukemia.