Estimated mean rates. The estimated mean rates (black lines) of successful naive CD4⁺ T-cell division, ρ(t) (A) and loss, δ(t) (B) as functions of age, t. The 2.5 and 97.5 percentiles are shown in gray. A proliferation rate of, for example, 0.02 means that approximately 2% of cells divide per day.