Compensatory Kdm4b expression has been observed in the testes of Kdm4d knockout mice, possibly representing an inverse effect.32 Further supporting the overlapping roles of the KDM4 family members, simultaneous deletion of Kdm-4a, -4b and -4c was required to determine the role of Kdm4 family members in a mouse model of MLL-AF9-driven leukemia.33 These reports indicate that KDM4B does not act in isolation to regulate gene expression, highlighting the need for systematic in-depth studies of transcriptional mechanisms. The gene discussed is KDM4D; the disease is leukemia.