The second example addresses the question of the relationship between genetic variants and phenotypes at an extremely immediate level, namely the correlation between DNA sequence variants and variation in the relative abundance of mRNA. Variation in transcript abundance is heritable and it is reasonable to expect that the variation in expression of some genes may correlate with psychiatric disease. Thus a gene expression profile could act as an endophenotype, although currently we do not know which genes show expression patterns correlated with psychiatric disease. Compared to any of the endophenotypes so far analysed (Table 1), variation in the amount of transcript is more proximal to DNA sequence variation and, if the assumptions about the nature of an endophenotype are correct, the genetic architecture of transcript variation should be relatively simple.
