A model for SCL's mechanisms of action in developmental hematopoiesis. Comparative analyses of the hematopoietic defects observed in SCL^−/− in vivo (conventional and conditional knockout, chimeras) and in vitro models (see “Introduction” for references) and in our DNA-binding mutant model allow to depict the steps in primitive and definitive hematopoiesis that do require SCL DNA-binding activity and, consequently, those that do not. The red thick bars represent the blocks in erythroid maturation that are documented in this report. The black thick bars represent blocks in specification or commitment that were reported in models in which SCL expression was totally abolished but not observed in this study. The dashed line shows the proposed contribution of primitive hematopoietic progenitors to definitive hematopoiesis.³⁵