SCL specifies hemangioblast development from early zebrafish mesoderm. Appearance of uninjected zebrafish embryos (a) and embryos injected with SCL mRNA at the 2-4 cell stage (b). SCL injected embryos developed a marked excess of hemoglobinized blood cells (arrowhead), a striking absence of blood circulation despite a beating heart and an abnormally curved axis. In situ hybridization experiments with SCL alone (c) or SCL and Flk-1 (d) showed a marked increase in cells coexpressing both endogenous SCL and Flk-1 in SCL-injected embryos. This increase was often unilateral (as illustrated in this figure), because the injected mRNA was frequently localized to one half of an embryo. NT, neural tube; NC, notochord; SM, somitic mesoderm; LM, lateral mesoderm. (Reprinted from Gering M, Rodaway ARF, Göttgens B, Patient RK, Green AR: The SCL gene specifies haemangioblast development from early mesoderm. The EMBO Journal, vol 17, p 4029, 1998, by permission of Oxford University Press.122)