been shown to be more heritable than the clinical diagnosis itself in some populations [Ehlers et al., 2004b]. Another phenotype that is common to many drugs of abuse is craving. Human and animal studies have demonstrated that craving is an important element in the addictive process and that control of craving may improve efforts at abstinence [see Wise, 1988; Robinson and Berridge, 1993; Anton, 1999; Sinha and O'Malley, 1999; Field et al., 2004; Heishman and Singleton, 2006; Haughey et al., 2008]. Evaluation of the heritability of cannabis withdrawal (h2 =0.28, p<0.001) and craving (h2 =0.36, p<0.0001) demonstrated that these two phenotypes were more heritable than the DSM-IV diagnosis of cannabis dependence (h2 =0.20, p<0.0001) in the UCSF family study population [Ehlers et al, unpublished work]. Additionally, five sites in the genome that provided evidence for linkage to these phenotypes were identified.
