Abstract: The present invention provides a method for diagnosing an autism spectrum disorder (ASD), or predisposition to develop an ASD, in a subject, which comprises the step of investigating a set of single nucleotide polymorphisms (SNPs) in a sample from the subject, wherein the number of SNPs in the set is such that the method can diagnose ASD with at least 70% accuracy. The invention also provides a kit for diagnosing an ASD, or predisposition to develop an ASD, in a subject, which comprises a plurality of primer pairs or probes capable of investigating such a set of SNPs in a sample from a subject, and a method for making such a kit.