Genetic risk factors with individually large effects are likely to be rare. Association-based methods to identify common genetic risk alleles in BD have met with limited success. Early studies implicated a few common variants with modest effects (Baum et al., 2008 and Ferreira et al., 2008). Robust support for one of these loci, the L-type calcium channel CACN1AC, has been obtained in a recent meta-analysis of 11,974 patients and 51,792 controls, along with new evidence for a second locus, ODZ4 (Sklar et al., 2011). However, the paucity of significant findings in very large samples of cases and controls suggests that the contribution of common genetic variants to heritability of BD is limited.
