Structure representation of each BMP receptor KO mutation and PCR genotyping strategy. Schematic diagrams showing the wild-type locus of the Alk2 (a), Alk3 (b), Alk6 (c), Bmpr2 (d), Acvr2a (e), and Acvr2b (f) gene, and the targeting vector for each gene. The positions (red arrows) of PCR primers for genotyping are indicated below the locus