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. Predominantly expressed in hematopoietic cells. Present in organs and tissues that contain stem and progenitor cells, myeloid and/or lymphoid: placenta, spleen, lymph nodes, thymus, bone marrow and fetal liver. Within the hematopoietic system, it is abundant in CD34(+) cells but undetectable in mature peripheral blood leukocytes, and its levels rapidly decrease during the differentiation of CD34(+) cells in response to hemopoietins. Uses different DNA- and protein-binding zinc fingers to regulate the distinct BMP-Smad and hematopoietic system. A chromosomal aberration involving ZNF521 is found in acute lymphoblastic leukemia. Translocation t(9;18)(p13;q11.2) with PAX5. The translocation generates the PAX5-ZNF521 oncogene consisting of the N-terminus part of PAX5 and the C-terminus part of ZNF521. Belongs to the krueppel C2H2-type zinc-finger protein family.