SLC6A4 and substance dependence: Polymorphisms in the second intron (STin2) of SLC6A4 gene are associated with the F4 question in the Fagerström Test for Nicotine Dependence, “How many cigarettes per day do you smoke?” Number of cigarettes was divided into four categories (“< 11”, “11–20,” “21–30,” and “> 30”) and transformed into Fagerström score points, 0, 1, 2, and 3.