We previously identified 18q21.33-q23 as a candidate region for bipolar (BP) disorder and constructed a yeast artificial chromosome (YAC) contig map. In a next step we isolated and analysed all CAG/CTG repeats from this region and excluded them from involvement in BP disorder. Here, in the process of identifying all CCG/CGG repeats from the region, we found a cluster of 6 genes encoding for serpins. For one of them, CAP2 located at 18q21.3, a mutation analysis was performed. Analysis of the thus identified single nucleotide polymorphisms (six in total) revealed a statistically significant association of SNP c.942G>T with BP disorder. It is thus an object of the present invention to provide a method of diagnosing BP or susceptibility to BP in an individual which method comprises determining, in a sample from the individual, the single nucleotide polymorphism in the CAP2 gene of the individual, and determining the status of the individual by reference to polymorphism in the CAP2 gene.