There are some limitations to our study, most of which relate to the sample composition. Because the sample consists of nuclear families ascertained for having at least two siblings with TS, generally excluding families who were bilineal for tics or OCD, there is little phenotypic variability with regard to tic diagnoses (only 14 sibs were unaffected for TS), and there are only a small number of families where both parents have either a tic disorder or OCD. Additionally, although some information is provided by the few half-siblings, the vast majority of families in this study were biological full siblings. Because we do not have diagnostic information on second and third degree family members, the heritability estimates are just that, estimates, as the VA component is not able to distinguish some genetic effects from environmental effects present within the family structure, potentially resulting in an over-estimate of the heritability if the shared environmental effects are substantial. Conversely, the decrease in variability for the TS phenotype may falsely decrease heritability estimates for TS and perhaps also for CMVT. Similarly, the fact our
