In previous studies, GABA system markers have been associated with a variety of phenotypes in a number of different contexts, many of which were not assessed in this study. For example, there is evidence that the influence of polymorphisms in the GABA system may vary with age or across developmental stages (Dick et al., 2006; 2009), and be moderated by environmental factors (Dick et al., 2009; Enoch et al., 2010). Interaction may also occur within and between GABA system genes, particularly among proximal or clustered genes (Uusi-Oukari et al., 2000). Markers in GABA system genes have also been associated with less-complex biological markers such as Beta-frequency EEG (Edenberg et al., 2004) and event-related potentials (Winterer et al., 2000) that meet the criteria to be considered endophenotypes more directly reflecting underlying genetic liability than their complex behavioral correlates (Begleiter and Porjesz, 2006).
