Assessment of developmental effects and myeloid cell numbers identifies DDX18-E76del a nonfunctional allele of DDX18 found in AML. (A-N) Lateral views, head to the left and dorsal upward. Boxed region shows area of corresponding close-up in the caudal hematopoietic tissue. In situ hybridization for mpx at 27 hpf in AB embryos injected with ddx18 morpholino (C-N) or control morpholino (A-B) along with control RNA (A-D) or DDX18 RNAs encoding WT (E-F) or sequence variant (G-N) proteins. Injection of DDX18 WT-, V621I-, V371-, or C183Y-encoding RNA was able to rescue the developmental and myeloid defects of ddx18 morphants (E-L compared with C and D). DDX18-E76del–encoding RNA was not able to rescue any of the ddx18 morphants (M-N) compared with panels C and D. (O) Western blot for Ddx18 showing the ddx18ATG/5′ UTR morpholino results in the loss of Ddx18 protein. (P) Western blot for DDX18 showing expression of sequence variant proteins. Control-injected embryos show expression because of cross-reactivity of the Ab with the zebrafish protein. (Q) Scatter plot showing number of mpx-expressing cells per embryo. ***P ≤ .0002. ns indicates not significant (Student t test).