Alcohol dependence in adults was found to be associated with a number of SNPs in the cholinergic M2 receptor gene (CHRM2) in two studies (Wang et al., 2004; Luo et al., 2005). A refinement of the study of Wang et al. (2004) showed that the association was present only in those subjects who had comorbid illicit drug dependence (Dick et al., 2007). This group of subjects and their family members form a genetically vulnerable group, that is, a group whose alcohol dependent members have a more heritable form of the disorder. The alcohol dependent members of this group had a significantly earlier age of onset of drinking compared to the alcohol dependent subjects without comorbid drug dependence. A generalized measure of externalizing psychopathology including alcohol dependence and illicit drug dependence is associated with the same group of SNPs in the CHRM2 gene (Dick et al., 2008). Additionally, there is variation in the genetic factors associated with alcohol dependence; multiple genetic factors were found to contribute to a DSM-IV diagnosis of alcohol dependence in adults (Kendler et al., 2012). Some differences
