Transcription factor that can both act as an activator or a repressor depending on the context. Involved in BMP signaling and in the regulation of the immature compartment of the hematopoietic system. Associates with SMADs in response to BMP2 leading to activate transcription of BMP target genes. Acts as a transcriptional repressor via its interaction with EBF1, a transcription factor involved specification of B-cell lineage; this interaction preventing EBF1 to bind DNA and activate target genes. Interacts with EBF1. Interacts with SMAD1 and SMAD4 (By similarity). Widely expressed. Expressed in all B-cell stages. Uses different DNA- and protein-binding zinc fingers to regulate the distinct BMP-Smad and hematopoietic system. Defects in Znf521 are a cause of B-cell lymphomas. The Znf521 gene is a frequent target of retroviral integration in murine B-cell lymphomas. Involved in most B-cell tumors in the AKXD-27 strain. Viral insertion into the Znf521 gene in the AKXD-27 strain causes Znf521 overexpression in B-cell tumors, resulting in the up-regulation of EBF1 and the increased expression of a number of EBF1 target genes. This in contrast to the role of Znf521 in other cells as a transcriptional repressor of EBF1. Misexpression initiates tumorigenesis by perturbing B-cell development via an interaction with EBF1. Belongs to the krueppel C2H2-type zinc-finger protein family.