It has often been argued that schizoaffective disorder may be closely related to schizophrenia – indeed, DSM–IV classifies schizoaffective disorder within diagnostic code category 295, as a subtype of schizophrenia. Thus, one important question is the following: is the enhanced number of hits seen in the RDC schizoaffective disorder, bipolar type group in our bipolar disorder data-set relatively specific to the RDC schizoaffective disorder, bipolar type group, or is it a general property of individuals with schizophrenia-like features (in which case it could be more usefully thought of as providing support for the genetic utility of schizophrenia). We have recently undertaken a genome-wide association study of schizophrenia using the same methodology as that used in the WTCCC study of bipolar disorder (same genotyping platform, same laboratory, same set of SNPs called at the same time using the same algorithm and with case–control comparisons made using the same set of 2938 controls).15 When we apply our analytic approach to our set of people with ‘schizophrenia spectrum’ (i.e. schizophrenia and schizoaffective disorders – the latter set includes the 279 individuals from the
