By using the yeast two-hybrid system, the zinc finger protein ZPR9 was identified as one of the B-MYB interacting proteins that associates with the carboxyl-terminal conserved region of B-MYB. ZPR9 was found to form in vivo complexes with B-MYB, as demonstrated by in vivo binding assay and coimmunoprecipitation experiments of the endogenously and exogenously expressed proteins. Deletion analysis revealed that this binding was mediated by all three functional domains, an amino-terminal DNA-binding domain, a transactivation domain, and a carboxyl-terminal conserved region of B-MYB. We show that the interaction of ZPR9 with B-MYB is functional because cotransfection of ZPR9 significantly up-regulates B-MYB transcriptional activity in a dose-dependent manner. In addition, coexpression of ZPR9 with B-MYB caused the accumulation of B-MYB, as well as ZPR9, in the nucleus. Furthermore, constitutive expression of ZPR9 in human neuroblastoma cells induces apoptosis in the presence of retinoic acid. These results strongly suggest that ZPR9 plays an important role in modulation of the transactivation by B-MYB and cellular growth of neuroblastoma cells.