The disclosure provides nucleic acids, including cDNA, comprising alterations that encode aspartic acid at a position corresponding to position 533 of the human G protein-coupled receptor 156 protein (GPR156). The disclosure also provides isolated and recombinant human GPR156 protein variants that comprise an aspartic acid at a position corresponding to position 533. The change to aspartic acid, and the gene encoding this change, associate with unipolar depression. The disclosure also provides methods for determining whether a subject has or has a risk of developing unipolar depression, based on the identification of such alterations in the gene (DNA or RNA) encoding GPR156.