The present invention provides a method of selection of a patient, who is a candidate for treatment with an NMDA antagonist drug, such as (S)-1-phenyl-2-(pyridin-2-yl)ethanamine or ketamine, whereby to predict an increased or decreased likelihood of response to the NMDA antagonist. The invention provides a method for determining the sequence of GABR-A2 at any of four single nucleotide polymorphism (SNP) sites known as rs3756007, rs11503016, rs17537359 or rs1372472. The method also provides ARMS primers optimised for determining the sequence at these GABR-A2 SNPs and diagnostic kits comprising suitable primers or probes for determining the particular SNPs.
