CNR1 and major depressive disorder: The cannabinoid receptor 1 gene (CNR1) on chromosome 6q14- 15 encoded CB1 protein that is likely to modulate the release of serotonin, norepinephrine and dopamine systems, disturbances of which are well documented in depression[19]and addictive disorders.[20] Several studies reported that SNPs of CNR1were associated with nicotine,[21] marijuana,[22] alcohol,[23] cocaine,[24] and poly-substance use disorder.[25] We demonstrate association between polymorphisms (rs806368) in the CNR1 gene and maximum dose of MMT and the risk of heroin addiction.