Distinct Notch1 requirement in developmental hematopoiesis. Notch1 is required only for the development of the long-term definitive hematopoietic stem cell compartment while dispensable for earlier, short-term definitive hematopoietic progenitors that are detected in embryoid bodies from in vitro ES cell differentiation and the early yolk sac/fetal liver in vivo. In contrast, the critical hematopoietic transcription factor SCL is required for the development of all hematopoietic progenitors, primitive and definitive,⁷²  and the transcription factor Runx1 is required for all definitive, but not primitive, progenitors.^46,59