HOXA9 is a transcription factor with a central role in both haemopoiesis and leukaemia. High levels of HOXA9 expression in haemopoietic cells is a characteristic feature of acute myeloid leukaemia (AML), and may be sufficient to cause this disease. Overexpression of Hoxa 9 markedly expands hematopoietic stem cells. HOXA9 expression changes dramatically with age - a uniformly low level of expression during early adulthood is replaced by a frequently very high expression in adults over sixty.