 The extra copies of key chromosome 21 genes underlie their increased susceptibility to AMKL by promoting the development of a certain type of inactivating mutation in the "GATA1" gene.The "GATA1" gene resides on the X chromosome and codes for two transcription factors, GATA1 and a shorter version, GATA1-S. GATA1 and GATA1-S contribute to regulating the expression of genes that control the maturation of megakaryoblasts to promegakaryocytes, megakaryocytes, and platelets as well as the maturation of erythroblasts to red blood cells.GATA1-S appears less active than GATA1 in controlling some of the genes that promote megakaryoblast maturation but more active than GATA1 in stimulating megakaryoblast proliferation.