Best fits of the single-exponential model and the multiexponential model to labeling experiments of different duration. At different time points during and after labeling, the percentage of labeled DNA of splenic (A) naive and (B-C) effector/memory (E/M) CD4⁺and CD8⁺ T cells was determined. Dots represent measurements (ie, individual mice) at different time points during labeling for 1, 4, and 8 weeks (●, black circles), and during delabeling after 1 week of labeling (⁠⁠, gray diamonds), 4 weeks of labeling (⁠⁠, gray circles), or 8 weeks of labeling (⁠⁠, gray triangles). (A-B) Data were fitted separately for each labeling period using the single-exponential (SE) model to estimate p of the cells and d* of the labeled cells for the corresponding labeling period. For naive T cells, a delay was added in the model as described in supplemental Materials equations 9 and 10 and was estimated to be 4 days (95% CI, 2-6 days). For effector/memory T cells (B), the best-fitting curves during the labeling period were not identical for the different labeling periods (indicated by arrows). (C) When the data were fitted separately for each labeling period using the multiexponential (ME) model (describing 2 kinetically different subpopulations; the addition of more subpopulations did not change the average turnover rate), the best-fitting curves during the labeling period were almost identical. Label enrichment was corrected for ²H₂O enrichment in plasma (supplemental Figure 1A) and was scaled between 0% and 100% by normalizing for the maximal percentage of labeled DNA as measured in thymocytes (supplemental Figure 1B).