CD4 and X-linked lymphoproliferative disease: Lastly, while several previous studies have investigated the function of CD8+ T cells in XLP [19],[20],[32], it is difficult to separate direct effects of SAP deficiency in these cells from indirect effects that may result from lack of “help” from either functionally impaired SAP-deficient CD4+ T cells or NK cells, or the absence of NKT cells, all of which can promote CD8+ T cell responses [33]–[36].