TRECs within different cell populations. (A) CD3⁺ T cells. (B) CD4⁺ T cells. (C) CD8⁺ T cells. In every panel, the bars indicate the 10th percentile levels for healthy children at different ages. The TREC content within the different cell populations is expressed as TRECs per million cells. TREC levels were variable but were generally lower than the 10th percentile levels for age-matched control samples.