We demonstrated previously that an α 1 -β 2 -γ 2 gene cluster of the γ-aminobutyric acid (GABA A ) receptor is located on human chromosome 5q34-q35 and that an ancestral α-β-γ gene cluster probably spawned clusters on chromosomes 4, 5, and 15. Here, we report that the α 4 gene (GABRA4) maps to human chromosome 4p14-q12, defining a cluster comprising the α 2 , α 4 , β 1 , and γ 1 genes. The existence of an α 2 -α 4 -β 1 -γ 1 cluster on chromosome 4 and an α 1 -α 6 -β 2 -γ 2 cluster on chromosome 5 provides further evidence that the number of ancestral GABA A receptor subunit genes has been expanded by duplication within an ancestral gene cluster. Moreover, if duplication of the α gene occurred before duplication of the ancestral gene cluster, then a heretofore undiscovered subtype of α subunit should be located on human chromosome 15q11-q13 within an α 5 -α x -β 3 -γ 3 gene cluster at the locus for Angelman and Prader-Willi syndromes.