In 2010, 1.7 million Americans used heroin or cocaine and about 1.35 million of these were dependent upon or abusing these substances (National Survey on Drug Use and Health, 2010). The risk for developing heroin or cocaine dependence is influenced by genetic factors, with heritability estimates ranging from 30–70% [21, 34, 35]. One gene that has been extensively studied in relation to drug addiction is OPRM1, the gene encoding the μ-opioid receptor (MOR). MOR is a seven transmembrane G-coupled protein receptor that exhibits high affinity for binding endogenous and exogenous opioids [30]. Mice lacking MOR have abolished therapeutic responses to opioids and display attenuated reward responses to cocaine [4, 16, 27]. In cocaine addicted men, positron emission tomography (PET) scans show increased MOR binding, which is associated with cocaine craving [42].
