The CLOCK gene, located within the 1-lod support interval of the linkage peak on chromosome 4, encodes one of many proteins that regulate circadian rhythm. Addiction to various drugs has been found to disrupt circadian rhythms [Johanson et al., 1999; Schierenbeck et al., 2008; Acuna-Goycolea et al., 2010]. The CLOCK gene also plays an important role in regulating dopaminergic transmission and cocaine reward [McClung et al., 2005], and circadian-related genes are implicated in cocaine-induced locomotor sensitization in the day/night cycle [Abarca et al., 2002].
