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Fig. S1 Valve formation is perturbed in santa. The sites of formation of the endocardial cushions in day 2 embryos of the ty219c san allele are indicated by staining with an in situ RNA probe to the neuregulin gene (Milan et al., 2006). (A,A′) Wild-type sibs of the san ty219c allele demonstrate distinct staining at the sites of future valve formation, indicated by the arrow. (B,B′) No staining is observed in the hearts of stage-matched san mutant embryos. The position of the atrial and ventricular chambers are outlined in A′ to better illustrate the location of the neuregulin staining. wt, wild type; −/−, homozygous mutant.