Such provocative questions have been raised by studies of trait-discordant monozygotic twins in relation to differential X-inactivation in the context of bipolar disorder (Rosa et al. 2008) and primary biliary cirrhosis (Mitchell et al. 2011), and differential DNA methylation at specific loci in the context of a caudal duplication anomaly (Oates et al. 2006), bipolar disorder (Dempster et al. 2011; Kuratomi et al. 2008), schizophrenia (Dempster et al. 2011), and BRCA1-related malignancy (Galetzka et al. 2012). This evidence concerns the gene BRCA1 and bipolar disorder.