PBX1/2/3/4 [F3]

The PBXs (Pre-B cell leukemia transcription factors) belong to the three amino acid loop extension (TALE) family of homeodomain containing protein. Human pre-B cell acute leukemias are frequently associated with a t(1;19)(q23;p13.3) chromosomal rearrangement, which creates a chimeric gene encoding a fusion between the E2A and PBX1 gene products. PBX2 and PBX3 share 92% and 94% respective identities with PBX1 over a 266 amino acid region flanking their homeobox domains, while all three proteins are quite divergent at their amino- and carboxy-termini. Two forms of PBX1 and PBX3 each differ primarily in their carboxy-termini and result from alternative mRNA splicing. Unlike other homeotic selector genes, which are expressed transiently during development and differentiation, PBX gene transcripts are ubiquitously expressed in both fetal and adult tissues and cell lines. PBX1 has two isoforms, PBX1a and 1b, that are divergent in sequence at the carboxy terminus, which is a result of alternative mRNA splicing. Additionally, PBX2 and PBX3 transcripts are detected in lymphoid cells, which do not express PBX1. PBX4 expression is confined to the testis, especially to spermatocytes in the pachytene stage of the first meiotic prophase.