Human Brain-Specific Angiogenesis Inhibitor 3 (BAI3) is a 177 kDa seven-span transmembrane (TM) protein, which is thought to be a member of the secretin receptor family. It is synthesized by neurons of the CNS and likely is a negative regulator of angiogenesis. BAI3 is 1498 amino acids in size. It contains three distinct regions: an N-terminal extracellular domain (ECD) (aa25-883), a 7-TM segment, and a C-terminal cytoplasmic region. The ECD contains four antiangiogenic TSP type 1 repeat (aa296-508), and one GSP domain (aa 816-867) that is likely used to cleave the ECD from the membrane-bound receptor. There is one altermate splice form that shows a deletion of aa 643-665. Over aa 25-880, human BAI3 shares 98% aa identity with mouse BAI3. BAI3 has been reported primarily in the brain, but is also localized to lung, testis, and pancreas. It might be involved in angiogenesis inhibition and suppression of glioblastoma.