Four clones have been isolated from the adult human brain cDNA library using an oligonucleotide probe corresponding to the first 20 amino acids of the brain amyloid polypeptide. The open reading frame of the sequenced clone coded for the known full amino acid sequence of the brain amyloid polypeptide. The 3 kb messenger RNA has been detected in a variety of tissues from human and many nonhuman species. The gene is highly conserved in evolution and has been mapped on human chromosome 21.