GATA1 and Dravet syndrome: The cellular and molecular events involved in initiation and evolution of TAM and ML-DS can best be understood as a three-step model which requires the presence within a fetal liver-derived haematopoietic stem or progenitor cell of (i) trisomy 21, (ii) an acquired GATA1 mutation, and (iii) at least one additional oncogenic mutation (Fig. 1).