help to address phenotypic heterogeneity. Endophenotypes for neuropsychiatric conditions can take a variety of forms, including rating scales, biomarkers, and neuroanatomical features. Our group has previously used and discussed the rationale for selecting Conners’ Continuous Performance test (CPT) measures as AD/HD endophenotypes (Kollins et al., 2008). The CPT is administered on the computer where a series of target and nontarget letters flash on the screen and the individual is instructed to press the spacebar as quickly as possible after he/she views a target letter. A series of measurements can then be obtained from these test results (Table 1). We, and others, have shown that some of these CPT-type measures are correlated with AD/HD symptoms or affection status (Epstein et al., 2003; Kollins et al., 2008) and are heritable (Groot et al., 2004; Kuntsi et al., 2006; Kollins et al., 2008). In our data set, heritabilities for these measures ranged from 28 to 57% (Kollins et al., 2008). Both these features, the correlation with affection status and having a heritable component, are important for selecting an appropriate endophenotype for genetic analysis (Castellanos and Tannock, 2002). Another approach to define endophenotypes is to focus on symptom counts, either in raw form, or
