Thymic output is decreased in patients with TBD. Thymus function was assessed by identifying CD31⁺CD45RA⁺ RTEs in stained PBMCs, and quantifying the copy number of TRECs in peripheral blood collected on Guthrie cards. KRECs were also evaluated in peripheral blood as indicative of B-cell maturation in bone marrow. Data suggest thymic hypofunction in patients with TBDs. (A) Percentages of CD4⁺ and CD8⁺ RTEs out of total CD4⁺ and CD8⁺, respectively. (B) Box plot of TRECs and KRECs copies in peripheral blood. (C-D) Spearman correlations between TREC copies and age in healthy individuals and patients with TBD, respectively. ∗P ≤ .05; ∗∗∗P ≤ .001. TBD, telomere biology disorder.