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Fig. 2 The sil mutant exhibits a loss of expression of ventricular contractile protein genes. Double staining with an atrial-specific antibody (S46) and an in situ RNA probe for ventricular myosin heavy chain (vmhc) (Yelon et al., 1999) was used to distinguish the atrial and ventricular chambers in wild-type (WT) (A, B) and silm656 mutant (-/-) (C-E) embryos. The hearts of wild-type (A) and silm656 mutant (C, D) embryos at 34 hpf express markers specific for the two cardiac chambers. However, by 52 hpf, the hearts of silm656 mutant (E) embryos no longer express the ventricle-specific vmhc marker while both chamber markers are still visible in wild-type embryos (B).