SCL, a transcription factor of the basic helix–loop–helix family, is a master regulator of
hematopoiesis. Scl specifies lateral plate mesoderm to a hematopoietic fate and establishes
boundaries by inhibiting the cardiac lineage. A combinatorial interaction between Scl and
Vegfa/Flk1 sets in motion the first wave of primitive hematopoiesis. Subsequently, definitive
hematopoietic stem cells (HSCs) emerge from the embryo proper via an endothelial-to-
hematopoietic transition controlled by Runx1, acting with Scl and Gata2. Past this stage, Scl …