Linkage of Bipolar Disorder to Chromosome 18q and the Validity of Bipolar II Disorder

Abstract
ONE OF the great challenges of research in psychiatry lies in the heterogeneity of the clinical entities.1 When clinical heterogeneity reflects a diversity of causes at the genetic level, the power to detect susceptibility loci by genetic linkage analysis is greatly reduced. As yet, there is no valid and reliable means of dividing clinical entities such as bipolar affective disorder (BPAD) into genetically simpler subtypes.