The UBCH5 isoforms ubiquitinate histone H2A in concert with PRC1 in vitro (44, 45), and TRIM37, an E3 ligase that associates with PRC2, ubiquitinates H2A in human breast cancer cell lines where RING1B of PRC1 is down-regulated (47). The gene discussed is UBE2D1; the disease is breast carcinoma.