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Fig. 4 Loss of gata6 causes defects in both linear heart size and regulation of anterior secondary heart field progenitor phenotype. (A-C) Representative whole-mount in situ hybridization images of 28 hpf gata6 sibling (A) and mutant (B) embryos analyzed with a probe targeting myl7. Scale bars: 100 μm. (C) Quantification of the length of the heart tube showed a significant reduction in the length of the myl7 expression domain in gata6-null embryos compared with siblings (paired t-test, P<0.01, data are mean±s.e.m.). Length was measured manually using FIJI software. (D-F′) Whole-mount in situ hybridization analysis revealed decreased expression levels of the outflow tract marker elnb (D,D′), and SHF markers spry4 (E,E′) and mef2cb (F,F′) from 1-3 dpf in gata6 mutant larvae compared with siblings. White arrows indicate areas of reduced mRNA expression levels in the mutants. Black arrows indicate areas of normal expression in the controls. Scale bars: 100 µm.