To study the relation between Gfi1 expression levels and myeloid leukemia, we have generated humanized GFI1 “knock in” expressing the Human GFI1 gene at WT levels (called “KI” mice) [38, 43] and mice expressing only a reduced level of GFI1 called “KD” [26, 41]. The gene discussed is GFI1; the disease is myeloid leukemia.