Four clones were isolated from an adult human brain complementary DNA library with an oligonucleotide probe corresponding to the first 20 amino acids of the beta peptide of brain amyloid from Alzheimer's disease. The open reading frame of the sequenced clone coded for 97 amino acids, including the known amino acid sequence of this polypeptide. The 3.5-kilobase messenger RNA was detected in mammalian brains and human thymus. The gene is highly conserved in evolution and has been mapped to human chromosome 21.