The Endothelin Receptor ETB is expressed in most tissues. Mutations of the receptor gene are responsible for Hirschsprung disease type 2, a multigenic disorder with phenotypes such as bicolored irides, megacolon, hypopigmentation, and hearing loss. The ETB receptor has similar affinity for all three endothelins and activates a phosphatidylinositol-calcium second messenger system. A splice variant, termed SVR, has been described; the sequence of the ETB-SVR receptor is identical to ET-B receptor, with the exception of the intracellular C-terminal domain. While both splice variants bind endothelin ET1, they exhibit different responses, which suggests that they may be functionally distinct. The endothelin B receptor has been reported to be expressed in adrenal, aorta, artery, brain, breast, eye, epididymis, ganglion, heart, kidney, liver, lung, ovary, placenta, prostate, skin, testis, uterus, and vessel. ESTs have been isolated from amnion, B-cell/lung/testis, breast, brain, colon, ear, embryo, eye, heart, kidney, liver, liver/spleen, lung, lymph node, placenta, prostate, and skin libraries.