BAI1 is an Orphan-B GPCR with an unknown ligand. BAI1 is postulated to be a member of the secretin receptor family, an inhibitor of angiogenesis, and a growth suppressor of glioblastoma. Along with BAI1, there are two other brain-specific angiogenesis inhibitor genes, designated BAI2 and BAI3, which have similar tissue specificities and structures. However, only BAI1 is transcriptionally regulated by p53. Loss of BAI1 expression has been implicated in glioblastoma. BAI1 expression has been reported primarily in brain, but it is also found in colon, lung, esophagus, and uterus. ESTs have been isolated from brain, lung, and placenta libraries.