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Fig. 8 Expansion of the cardiac Bmp expression domain in the absence of Hey2 homologs in mouse and zebrafish embryos. (A,B) Bmp2 expression marks the AVC and IC of E9.5-E10.0 mouse wild-type (WT) embryos (A), and this region is expanded in homozygous Hey2 mutant siblings (B). Arrows indicate the borders of Bmp2 expression. (C) Measurements of the atrioventricular length of the Bmp2 expression domain relative to head length revealed a 1.7-fold expansion in mutants (n=5) relative to wild type (n=5; P<0.005, Student's t-test), compiled from two litters. Error bars correspond to standard deviation. (D,E) Bmp4 marked the developing AVC (gray arrowheads) in 48 hpf wild-type zebrafish embryos (D), whereas gridlock (grl) mutant embryos (E), which lack the Hey2 homolog, showed diffusely distributed Bmp4 in the AVC and ventricular regions (red arrow), and strong ectopic expression in the inflow tract (blue arrow), as diagrammed in schematics to the right.