We demonstrated previously that an α1-β2-γ2 gene cluster of the γ-aminobutyric acid (GABAA) 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 α4 gene (GABRA4) maps to human chromosome 4p14-q12, defining a cluster comprising the α2, α4, β1, and γ1 genes. The existence of an α2-α4-β1-γ1 cluster on chromosome 4 and an α1-α6-β2-γ2 cluster on chromosome 5 provides further evidence that the number of ancestral GABAA 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 α5-αx-β3-γ3 gene cluster at the locus for Angelman and Prader-Willi syndromes.
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