BMI1 (B lymphoma Mo-MLV insertion region 1 homolog) is a core component of the polycomb group (PcG) multiprotein PRC1 complex which plays an important role in transcriptional repression of gene expression during development. BMI1 association with RING protein 1B is important to the E3 ligase activity of RING1B in the PRC1 complex. BMI1 is an oncogene whose overexpression is associated with many types of human tumors and is a stem-cell marker and regulator of stem cell self-renewal.