The number of presumptive hematopoietic stem cells is reduced in clo mutant embryos. Nomarski micrographs of living wild-type (A), and clo mutant (B) embryo around the 20-somite stage. Hematopoietic stem cells are thought to come from the posterior region of the ICM. Wild-type embryos contain a large number of round, undifferentiated cells in this region (open arrows) while clo mutants exhibit a marked reduction in the number of these cells. These presumptive stem cells are further identified as they express GATA-2 but not GATA-1 (see Fig. 7). This reduction of cells in the posterior region of the ICM is the earliest morphological phenotype observed during development and was used reliably to sort mutant embryos at an early stage.