There are 5 additional candidate genes located in chromosomal regions showing association with SC (Table 1). Among these, SLC9A9 encodes a sodium and hydrogen exchanger in chromosome 3q24, and is of particular interest as it has been linked to tobacco smoking initiation, a behavior highly comorbid with alcohol use42. Studies have repeatedly shown evidence of association between SLC9A9 and attention deficit hyperactivity disorder (ADHD)43–45. Because inattention is a predictor of smoking initiation46 and ADHD and smoking are co-transmitted through families more often than expected by chance47, it is possible SLC9A9 influences both smoking and inattention. The association of SLC9A9 with SC identified in this study suggests a genetic connection between ADHD, smoking and alcoholism. Our preliminary data showed that SLC9A9 mRNA expression is detectable in human frontal cortex. Total mRNA expression in brain tissues derived from alcoholics is 1.09 fold higher than in the brain tissues derived from non-alcoholic subjects. Another gene of interest, CXCR7 in chromosome 2q37.3, encodes C-X-C chemokine receptor type 7, a member of the G-protein coupled receptor family. Our preliminary assay with human frontal cortices showed
