Previously, the remaining wild-type MLL allele in cancer cells was shown to be critical for leukemogenesis induced by MLL fusion (47); however, a recent study reported that MLL2/KMT2B, another trithorax family methyltransferase that is most closely related to MLL/KMT2A (48), sustains growth of MLL-rearranged leukemia and represents a more relevant drug target (49). The gene discussed is KMT2A; the disease is cancer.