Within the Nurses’ Health Study (NHS) [10], Nurses’ Health Study II (NHSII) [11], Health Professional Follow Up Study (HPFS) [12] and the Physicians’ Health Study (PHS) [13], since 2007, we have, conducted twelve GWAS of different traits including type 2 diabetes [14], coronary heart disease [15], several cancer types [16–19] and mammographic density [20,21]. In total, we have assembled GWAS data for 20,769 individuals across the cohorts, creating unprecedented opportunities to conduct secondary analyses on other collected outcomes. Indeed, we have used one or many of these GWAS to analyze secondary phenotypes including but not limited to body anthropometrics [22–24], hair color [25], reproductive aging [26], smoking behavior [27], telomere length [28], mammographic density [29], cutaneous nevi [30], melanoma [30], depressive symptoms [31], coffee consumption [32] as well as circulating levels of B12 [33], folate [34], hormones [35], vitamins [36,37], retinol [38] and e-selectin [39]. However, GWAS of secondary traits face practical issues in terms of different genotyping arrays, low variability in the phenotype of interest within a single GWAS (e.g. rare diseases where only a handful of cases may
