Previous experience in transfer of WT or lentivirus-corrected Prf−/− bone marrow in the murine model, as well HSCT results in patients with HLH, suggests that engraftment of 20% donor (or autologous functional) perforin is enough to achieve protection from HLH.15, 16, 27 However, in our studies using gene-corrected CD8 T cells, we saw that 8% to 15% engraftment at the time of LCMV infection was able to protect against LCMV-induced HLH. The gene discussed is CD8A; the disease is hemophagocytic syndrome.