SNP-based P-values were obtained in two association analyses of initiation conducted in a sample comprising 6744 participants. Two alternative reference panels—the 1000G and the GoNL, respectively—were used to impute genotypes in our sample. Owing to a better imputation quality (The Genome of the Netherlands 2014), the association signals in the GoNL imputed genotype data were slightly stronger than those obtained based on the 1000G imputed SNPs.1 Consequently we took forward these results for the gene-based tests. The P-values for the 5 896 100 GoNL SNPs showed no inflation i.e., the lambda inflation factor equaled 1.019, where a value of 1 indicates no deviation from the expectation of the observed test statistic due to effects of population stratification. The quantile–quantile plot is given in Supplemental Figure S2. The most strongly associated SNP was the low frequency GoNL SNP rs35917943 (MAF < 5 %; P = 1.6 × 10−7). The region harboring this SNP is displayed in Supplemental Figure S3 (Pruim et al. 2010). Supplemental Table S2 contains the top SNPs associated with initiation at P < 1 × 10−5. Table 1
