OBJECTIVE—HbA1c (A1C) is substantially determined by genetic factors not shared in common with glucose. Fractions of the variance in A1C, the glycation gap (GG; previously called the glycosylation gap) and the hemoglobin glycosylation index, correlate with diabetes complications. We therefore tested whether GG (measured A1C − A1C predicted from glycated serum proteins [GSPs]) was genetically determined and whether it accounted for the heritability of A1C. RESEARCH DESIGN AND METHODS—We conducted a classic twin study on A1C and GSP collected in 40 and 46 pairs of monozygotic and dizygotic healthy female twins, respectively. The predicted A1C was based on the regression line between A1C and GSP in a separate population spanning the pathophysiologic range. RESULTS—GG was more strongly correlated between monozygotic (r = 0.65) than dizygotic (r = 0.48) twins, adjusted for age and BMI. The best-fitting quantitative genetic model adjusted for age and BMI showed that 69% of population variance in GG is heritable, while the remaining 31% is due to unique environmental influences. In contrast, GSP was similarly correlated between monozygotic (r = 0.55) and dizygotic (r = 0.49) twins, hence not genetically determined. GG was strongly correlated to A1C (r = 0.48), attributable mostly to genetic factors. About one-third of the heritability of A1C is shared with GG; the remainder is specific to A1C. CONCLUSIONS—Heritability of the GG accounts for about one-third of the heritability of A1C. By implication, there are gene(s) that preferentially affect erythrocyte lifespan or glucose and/or nonenzymatic glycation or deglycation in the intracellular, rather than extracellular, compartment.