One of the most interesting as well as challenging observations has been the degree of phenotypic variability associated with individual CNVs, i.e. the “expressivity” of the genotype. Virtually every CNV allele that is associated with a psychiatric disorder is present at a low frequency in populations of healthy controls, and virtually every CNV is also associated with a wide variety of other neuropsychiatric or neurodevelopmental conditions including bipolar disorder, seizure disorder, intellectual disability, attention deficit hyperactivity disorder (ADHD) etc (Cooper et al., 2011; Elia et al., 2011; Girirajan and Eichler, 2010; Sahoo et al., 2011; Williams et al., 2011). Several examples of variable expressivity of CNV genotypes are described in Table 1
