To test this hypothesis, we examined laterality defects in mice carrying mutations in both ACVR2B and inversus viscerum (iv) genes, because iv(-/-) mice display a spectrum of laterality defects, including situs inversus, right isomerism, and left isomerism. ActRIIB mRNA was also found in brain, spinal cord, and ganglion, but usually appeared earlier in development than the ActRII message. Retinoic acid induces activin receptor IIB mRNA in F9 embryonal carcinoma cells.