The influence of the length of the labeling period on the estimated turnover rate p. Consider an artificial long-term labeling experiment of a kinetically heterogeneous cell population, in which the labeled fractions of a slow (dark gray) and a fast subpopulation (light gray) gradually increase with the duration of label administration (A). During labeling, samples are obtained, and the percentage of labeled DNA is determined at several time points (B, black circles). During the labeling phase, the initial accrual of label (the slope nearby the origin, as indicated by the black tangent line) reflects p of the kinetically heterogeneous population (A, situations 1 and 2; B, white area). If labeling is continued, the enrichment level of the fastest subpopulation may start to saturate (A, situation 3). Although cells of the fastest subpopulation continue to divide after this point, this is no longer reflected by a corresponding increase in their enrichment level. If sampling is continued (B, gray area), any further increase in labeled DNA is largely the result of cell production in the slow subpopulation, reflected by a second, flatter slope of the labeling curve (B). If the label enrichment data are fitted using a single-exponential model (dotted black curve), the model seeks a compromise between the initial, steep increase and the later, slower increase of label enrichment. As a result, the model fit is forced to bend downward from the initial slope, and the average turnover rate will become increasingly underestimated with increasing duration of label administration. In contrast, the multiexponential model corrects for this effect by allowing for multiple slopes during the labeling phase (B, dashed black curve), and thereby yields a reliable estimate of the average turnover rate, independent of the length of the labeling period.