Loss of telomere repeats with age in leukocytes. Adapted, with permission, from Baerlocher et al41 and Aubert et al.42 (A-B) Telomeric DNA in human granulocytes (Gr, n = 808) and naive T cells (nT, n = 832) from normal human donors is lost most rapidly in the first year of life, slows down after puberty, and is more pronounced in T cells than in granulocytes. The range in distribution of telomere length at a given age is expressed as a percentile based on best fit regression lines: 99th (red), 90th (green), 50th (black), 10th (green) and 1st (blue). The shaded blue area reflects ∼3 kb of subtelomeric DNA included in calculated values to allow comparisons with telomere length estimates by terminal restriction fragment (TRF) analysis. (C) The median telomere length in human granulocytes and naive T cells in female (pink; n = 29) vs male (blue; n = 29) cord blood samples. (D) The decline in telomere length in nucleated blood cells from a baboon was nonlinear and showed a pronounced drop after 1 year. (E) In humans, naive T cells from females (n = 414) have, on average, longer telomeres compared with those in males (n = 418). (F) Patients heterozygous for a mutation in either TERT or TERC (n = 58) have very short telomeres compared with siblings (n = 37) without the mutation.