hdac1 acts between notch and runx1 in HSC specification. Whole-mount in situ hybridization of embryos between 36 and 40 hpf. Embryo manipulation, genotype, and probes used are described for each panel. Red arrowheads represent arterial expression of HSC markers; blue arrowhead, venous expression of HSC markers. Lateral views, anterior left; low magnification (original magnification ×10) and higher magnification (original magnification ×40) of the trunk region. Overexpression of NICD does not rescue the hdac^−/− HSC phenotype (A-D). After heat shock (“Methods”), control hdac1 mutant siblings and hdac1^−/−;Tg(hsp70:gal4);Tg(uas:nicd) embryos fail to induce c-myb (A) or runx1 (B) expression, whereas Tg(hsp70:gal4);Tg(uas:nicd) embryos show expanded c-myb and runx1 expression (Figure 3A,B). Overexpression of runx1 RNA expands c-myb–expressing cells in wild-type (compare panel F with panel E) and partially suppresses the hdac^−/− HSC phenotype (G,H).