The aim of our analysis was to use the WTCCC bipolar disorder genome-wide association data-set to determine whether any of the bipolar-spectrum diagnostic subsets within the sample provided enhanced genetic utility, compared with the bipolar disorder sample as a whole and compared with the other diagnostic subsets. Of the four RDC and three DSM–IV diagnostic categories examined, only the RDC schizoaffective disorder, bipolar type category showed a significantly enhanced number of association signals above expectation based on the null hypothesis that all the diagnostic subsets were simply a random selection from the bipolar disorder sample as a whole, i.e. that the genetic contributions do not vary according to the diagnostic subset. It is important to stress that this null hypothesis allows for there being genetic differences between participants with bipolar disorder and controls – it is not a null hypothesis that there are no genetic effects operating in bipolar disorder. Thus, our analysis provides evidence that, within our data-set and at the association threshold considered (i.e. λ-corrected P<10–5), the RDC schizoaffective disorder, bipolar type sample is a particularly valuable phenotype
