Allo-transplant patients have impaired thymus function. Thymic activity was assessed in patient and donor samples using the TREC assay and represented as TREC copy number per 10⁵ CD3⁺ T cells. Box plots define the values for median, range, 25th, and 75th percentiles. One outlier with a value of 1292 in the group of no-cGVHD patients is not shown in the figure. P values were calculated using the Wilcoxon rank-sum test; cGVHD versus no cGVHD, P = .13; cGVHD versus healthy donors, P < .001; no cGVHD versus healthy donors, P = .02.