cTfh cells are also quantitatively and qualitatively reduced by mutations in CARMIL2 (encoding RLTPR; Fig. 2), which causes a CID with clinical features including hypogammaglobulinemia, poor humoral/Ag-specific immune responses, and few memory B cells (Lévy et al., 2023; Wang et al., 2016). Here, CARMIL2 is linked to combined immunodeficiency.