Figure 1. Flowchart for developing DNAm GrimAge. Surrogate DNAm-based biomarkers for smoking pack-years and plasma protein levels were defined and validated using training and test data from the Framingham Heart study (stage 1). Only 12 out of 88 plasma proteins exhibited a correlation r >0.35 with their respective DNAm-based surrogate marker in the test data. In stage 2, time-to-death (due to all-cause mortality) was regressed on chronological age, sex, and DNAm-based biomarkers of smoking pack-years and the 12 above mentioned plasma protein levels. The elastic net regression model automatically selected the following covariates: chronological age (Age), sex (Female), and DNAm based surrogates for smoking pack-years (DNAm PACKYRS), adrenomedullin levels (DNAm ADM), beta-2 microglobulin (DNAm B2M), cystatin C (DNAm Cystatin C), growth differentiation factor 15 (DNAm GDF-15), leptin (DNAm Leptin), plasminogen activation inhibitor 1 (DNAm PAI-1), tissue inhibitor metalloproteinase 1 (DNAm TIMP-1). The linear combination of the covariate values XTβ was linearly transformed to be in units of years. Technically speaking, DNAm GrimAge is a mortality risk estimator. Metaphorically speaking, it estimates biological age.