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). This evidence concerns the gene BMI1 and leukemia.