SCL is required for development of the 3 hematopoietic waves at specification and maturation stages. Specification, development, and maturation stages of hematopoietic waves 1, 2, and 3 during mouse development are depicted.⁶  A common origin for waves 1 and 2 in the early epiblast is shown, but this is still the subject of debate (see “SCL confers hematopoietic fate to mesodermal/endothelial precursor cells” and Box 2). The (A) YS and (B) aorta-gonad-mesonephros (AGM) waves are shown as independently specified, as established in Xenopus embryos through elegant lineage-tracing studies.⁷  However, the origin of the angioblasts giving rise to wave 3 is not yet established in higher vertebrates. The SCL-dependent cellular transitions are represented by bold arrows. The hematopoietic lineages described as dependent on SCL activity for terminal maturation in the fetal liver are in bold font. The main features of SCL’s activities in specification and maturation stages are summarized below the diagram. E5.5-E12.5, embryonic days E5.5-E12.5; EHT, endothelial-to-hematopoietic transition; Ery^D, definitive erythroid cells; Ery^P, primitive erythroid cells; Mk, megakaryocyte; PS, primitive streak.