Fig. 1. Diminished Frs2 expression in Frs2^cn outflow tract and second heart field. Expression of Frs2 in (A) 0-somite stage (ss) or (B) E9.5 mouse embryos was assessed by immunostaining with anti-FRS2 antibody (green) on paraffin sections. Nuclei were stained with To-Pro3 (a DNA stain, red). a1-3, b1-3 and c1-3 are high-magnification views of the boxed areas in a, b and c, respectively. Note that in Frs2^cn/Nkx mutants, Frs2 expression was diminished in ventricular myocardium (V-M), outflow tract myocardium (OFT-M), endocardium (OFT-E), pharyngeal endoderm (PE) and splanchnic mesoderm (SM). In Frs2^cn/Mef mutants, Frs2 expression was disrupted in V-M, OFT-M and SM, but was intact in the OFT-E and PE. f, floxed allele.