used phenotype for ND based on self-reported smoking behaviors. The second data used a single item of the FTND questionnaire, “How soon after you wake up do you smoke your first cigarette”, or time to smoke the first cigarette (TFC) as a trait. This question can be seen as a measure of nicotine withdrawal since the half-life of nicotine in the human body is about 2 hours67. Smokers often experience nicotine withdrawal in the morning after not smoking overnight. The third data65 used the plasma cotinine concentration (COT) as a trait. Cotinine is the major metabolite of nicotine, and its half-life is much longer than that of nicotine. Therefore, its concentration in plasma can be considered an index of nicotine intake in recent days6869. Because the quantity of nicotine intake is one of the most important measures of ND, COT may be considered a measure of ND as well. In these studies, FTND, TFC and COT were treated as quantitative traits. The sample size for FTND was 16,237, excluding the Netherlands Twin Registry sample because some of its subjects were also used in COT GWAS. The sample sizes for TFC and COT were 15,705 and 4,575 respectively. The FTND and
