Figure 1. The DNA Methylation Clock: How It Works.

(A) The trajectory of change with age of methylation of eight CpGs, four with negative coefficients (left) and four with positive coefficients (right), colored by the rate of change with age. Darker colors indicate faster rates of change and, thus, stronger weights (numerically larger coefficients) in the epigenetic clock.

(B) Eight clock CpGs (columns) in three individuals of different chronological ages (rows) whose methylation values at each CpG are indicated by shading of the filled circle (fractional methylation, 0.0–1.0). The box color represents the coefficient of change over age (slope of line, top). In each colored box, the numerical product of the methylation value and the associated coefficient are shown. Summing across all boxes together with the intercept learned during clock building results in an epigenetic age that approximates chronological age.