Cord blood TREC levels during the third trimester of gestation. Cord blood samples were collected from 45 newborns of 26 to 41 weeks gestational age. Peripheral blood lymphocytes were harvested from 12 young adults. Lymphocytes were separated into CD4⁺ and CD8⁺ subsets using magnetic beads, and TREC concentrations were determined by quantitative PCR. TREC levels are presented as box plots showing medians, 25th and 75th percentiles are presented as boxes, and 10th and 90th percentiles are presented as whiskers. Percentages of CD4⁺ and CD8⁺ T cells expressing TRECs were approximately 6-fold and 4-fold higher, respectively, in full-term newborns (filled boxes) than in the young adults (open boxes) (A). TREC levels in the CD4⁺ T-cell subset were plotted in relation to the gestational age of the infant. Percentages of TREC⁺CD4⁺ T cells remained stable throughout the third trimester (B).