Ash2 is a component of the Set1/Ash2 histone methyltransferase (HMT) complex. This complex specifically methylates K4 of histone H3, thereby activating transcription. Methylation of K4 is blocked by premethylation of the neighboring K9, a repressor of transcription. This indicates that the Set1/Ash2 HMT complex mediates the crosstalk between K9 methylation and K4 methylation. Ash2 plays a role in hematopoiesis and may be associated with some kinds of leukemia.