Samples
Peripheral blood leukocyte DNA was bisulphite converted and DNA methylation profiling was performed using the Illumina HumanMethylation450K platform (Illumina, San Diego, CA). Samples from patients treated with 6-MP or placebo were randomly distributed across chips. A total 41 technical replicates were distributed across chips, runs, and cohorts. Genotype analysis was performed using the Illumina Omni Express Exome (500k SNPs) array for the TOPPIC cohort and the Illumina CoreExome Beadchip array.