It has been suggested that the effort to identify genetic factors and the mechanisms whereby they influence addiction may be aided by the use of phenotypes that may be more closely related to the biological processes underlying risk for use disorders [Gottesman and Gould, 2003]. One phenotype that most DSM-IV substance dependence syndromes have in common is a withdrawal syndrome defined as a combination of physiologic and psychological processes that occurs after cessation of its use [Edwards, 1986; Weisbeck et al., 1996; Hasin et al., 2006, 2008]. Although withdrawal is not required to meet DSM-IV criteria for drug/alcohol dependence it is associated with a more severe clinical course and a poorer prognosis [Schuckit et al., 1999; Budney et al., 1999, 2003, 2004, 2008; Budney and Hughes, 2006; Hasin et al., 2000, 2008; Ehlers et al., 2004a]; and as such, has 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
