Although there was no significant correlation between these three assays in the healthy controls, the SI value for <10% [3H]-thymidine proliferation in those with T cell deficiency was compatible with CFSE- and Ki-67-stained lymphocyte percentages, and validated in patients with IL2RG, RAG1, and ZAP70 mutations. Here, MKI67 is linked to congenital T-cell immunodeficiency.