CD4 and MHC class II deficiency: Unlike patients with classical type II BLS, who typically show low (<20%) frequencies of CD4+ T cells within the CD3+ T cell compartment — including 1 previously studied patient who carried a defective CIITA mutation (BLS01; ref. 40) — we found that patients BLS02 and BLS03 had 38.5% and 63.7% CD4+ T cells, respectively, among their circulating T cells (Supplemental Figure 7A and ref. 41).