GSTM1 and Schnyder corneal dystrophy: NGSP, National Glycohemoglobin Standardization Program; Hb, hemoglobin; PCA, principal component analysis; FG, fasting glucose; PEA, percent of European ancestry; GWAS, genome-wide association studies; NHWs, non-Hispanic Whites; T2DM, Type 2 diabetes mellitus; OGTT, oral glucose tolerance test; SCD, sickle cell disease; SCT, sickle cell trait; YOP, year of publication; SE, standard error