Thus, flow cytometry has (i) accurately defined the cell types that express SAP (predominantly T, NK, and NKT cells, but rarely B cells), (ii) offered a rapid and sensitive diagnostic tool to detect not only XLP patients whom lack SAP expression, but also female carriers of the XLP trait who have bimodal SAP expression in their T and NK cells (Figure 2) (57), and (3) revealed lymphocyte defects in XLP-1, thereby providing insight into disease pathogenesis. This evidence concerns the gene SH2D1A and X-linked lymphoproliferative disease.