Expression pattern of zebrafish and human ddx18. (A-K) WISH for ddx18 in WT embryos, from 1 cell to 48 hpf. Lateral view, head to the left (B-K). (A) ddx18 is expressed at the 1-cell stage indicating it is maternally supplied. (B-C) Ubiquitous expression is observed during early development. (D) Expression in the ICM region of the 24-hpf embryo is indicated by the arrowhead. Close-up view is shown in panel E. (F) Expression of ddx18 becomes more localized to the head and along the trunk in the region of the aorta (arrowheads) at 48 hpf. (G) Ventral view of expression in blood cells over the yolk. (H-K) transverse cryosections from WISH embryos at 48 hpf, imaged on a Zeiss Axio imager Z1 microscope using a Zeiss 63×/1.4 NA Apochromat oil lens (Carl Zeiss) and Openlab software (PerkinElmer). The levels of the sections are indicated in panel F by dashed lines. (H) ddx18-expressing cells are found in the wall of the aorta, which are likely to be HSCs, the neural tube is denoted in this view (nt). Close-up view of the boxed region is shown in panel I, indicating expression of ddx18 in cells within the wall of the aorta (ao; arrows), compared with the cardinal vein (vn;, arrowhead). ddx18 expression is also noted in the gut (g) and notochord (n). (J) ddx18 is expressed in cardiac muscle. Close-up view of boxed regions in J shown in K identifies the atrium (a) and ventricle (v). (L-M) WISH for ddx18 in ddx18^hi1727 siblings (L) and homozygous mutants (M) show the viral insertion results in loss of ddx18 transcript. (N) Quantitative PCR for DDX18 in primary human CB selected for CD34⁺ on day 0 (D0) and at 2 days and 7 days following liquid culture in myeloid or erythroid media. DDX18 is expressed in all cell subsets is highest in myeloid culture after 7 days. Expression is shown relative to β actin and normalized to expression in CD34⁺ cells at day 0. e indicates eye; a, atrium; v, ventricle; nt, neural tube; n, notochord; g, gut; ao, aorta; and vn, vein.