When participants were 20.5 years old, a certified phlebotomist drew four tubes of blood (30 ml) from each participant and shipped it the same day to a laboratory for preparation. Typical DNA yield for each pellet was between 10 and 15 mg of DNA. The Illumina (San Diego, CA) 450K HumanMethylation Beadchip was used to assess Genome-wide DNA methylation. The average correlation coefficient between replicate samples was 0.99 and data were cleaned using the Illumina Genome Studio Methylation Module, Version 3.2. Quantile normalization (e.g. Pidsley et al., 2013) was used to correct distributional problems inherent in array data using the watermelon (2013) R package (Team, 2012) and the “dasen” function recommended by Pidsley et al. (2013). Following quantile normalization, no technical variability (chip/batch effects) remained in array data.
