Longitudinal changes in naive CD4⁺ T-cell counts and CD4⁺ TREC numbers between early and late follow-up. Naive CD4⁺ T-cell counts (A) and CD4⁺ TREC numbers (B) were followed at early (1-4 years after HSCT) and late (> 5 years after HSCT) follow-up. Black circles and gray triangles denote patients with SCID with poor and good reconstitution of the naive CD4⁺ T-cell population at late follow-up, respectively. Open circles denote control values which were taken from age-matched (1-4.5 years and 5-32 years old, respectively) healthy individuals. The 2 open triangles in panel B represent the only child with good long-term immune reconstitution despite low TREC numbers at early follow-up. During early follow-up, this child was in an extremely poor clinical state and had malabsorption, which may explain the abnormal immune pattern at this time point.