GATA1 and transient myeloproliferative syndrome: However, although T21 is crucial for preleukaemia initiation, a second hit represented by in utero acquisition of somatic mutations of GATA-1 is required for the onset of a preleukaemia condition, referred to as transient abnormal myelopoiesis (TAM), previously also known as DS-related transient myeloproliferative disorder (TMD-DS), that occurs in 5–10% of newborns with DS and is strictly dependent on the coexistence of T21 and GATA-1 mutations [22,23].