PRC1 and leukemia: Initial reports investigating the role of BMI1 using BMI1 knockdown and/or MLL-AF9 leukemias generated on a Bmi1−/− background suggested that PRC1 canonical function is not required for MLLr leukemogenesis, although some transcriptional and minor functional effects on leukemia initiating cell frequency were observed (66, 217, 218).