FOXP3 and X-linked lymphoproliferative disease: The ability to quantify expression of SAP, XIAP, BTK, FOXP3, and DOCK8 proteins by intracellular staining and flow cytometric analysis expedited detection and diagnosis of patients with X-linked lymphoproliferative disease (XLP) type 1 (LOF mutations in SH2D1A), XLP-2 (LOF mutations in XIAP/BIRC4), XLA (BTK), IPEX (FOXP3), and an AR form of HIES (DOCK8), respectively (51–56) (Figure 2).