This gene contains a single nucleotide polymorphism that causes an amino acid substitution within the 11 of 12 ankyrin repeats of ANKK1 (Glu713Lys of 765 residues). This polymorphism, which is commonly referred to Taq1A, was previously believed to be located in the promoter region of the DRD2 gene, since the polymorphism is proximal to the DRD2 gene and can influence DRD2 receptor expression. It is now known to be located in the coding region of the ANKK1 gene which controls the synthesis of dopamine in the brain. The A1 allele is associated with increased activity of striatal L-amino acid decarboxylase.