Their results may conform with our findings since it provides biological plausibility for our finding of about two-times higher frequency of the all-variant TT-TT-TT haplotype in non-smokers since ABCB1 has been implicated in the transportation of endogenous compounds such as opioid peptides, steroids, glutamate, and endorphin, which act as neuronal modulators in the central nervous system, and which may play roles in mechanisms of substance dependence (Oude Elferink and Zadina, 2001; Choi, 2005; Zhou, 2008). The gene discussed is ABCB1; the disease is substance dependence.