However, while circulating T follicular helper (cTfh) cells—the CD4+ T cell subset that guides B cell differentiation [17]—have been reported to be present in normal or increased proportions in DADA2 patients, ADA2-deficient CD4+ T cells apparently exhibit features of impaired help for B cell differentiation such as reduced IL-21 production and CD40L expression in vitro [16]. The gene discussed is CD4; the disease is deficiency of adenosine deaminase 2.