Whole-mount in situ hybridization. Expression of ANF (a and b), MLC2v (c and d), MLC2a (e and f), Hand1 (g–n), Hand2 (o and p), and Nkx2.5 (q–t) was detected. (a–l and o–r) 9.5 dpc; (m, n, s, and t) 8.5 dpc; (a and b) Right lateral views; (c–i, k, q, and r) left lateral views; (j, l–p, s, and t) frontal views. ANF expression in the right ventricle is reduced in the Rae28^–/– embryo (b), while MLC2v expression extends into the right atrium (d). Expression of Hand1 was markedly reduced in the hearts of the Rae28^–/– embryos (h, k, and l). Since AVC and IVS formations were incomplete in the Rae28^–/– embryos, the portions corresponding to AVC and IVS are indicated by an arrowhead and a dotted line, respectively (k and l). Note that Nkx2.5 and Hand1 expressions were markedly reduced in the Rae28^–/– embryos at 9.5 dpc (k, l, and r) but were unaffected at 8.5 dpc (n and t). RA, right atrium; LM, lateral mesoderm; AVS, AV sulcus; AT, atrium; P, pharyngeal arch; H, heart. Scale bar: 200 μm.