TK⁺-cell add-backs prompt the systemic release of IL-7, leading to T-cell immune recovery. Line graphs indicate the serum concentration of IL-7 (solid lines) and the absolute T-cell counts (dashed lines) in 3 representative TK007 patients who experienced systemic engraftment of TK⁺ cells and subsequent immune reconstitution (3 of 12 studied patients are shown in the left panels) and in 2 of the 3 studied patients who received the suicide gene–modified cells but failed to attain T-cell reconstitution (right panels). Arrows indicate the time of HSCT (white), infusions of TK⁺ cells (black), or infusions of unmanipulated donor lymphocytes (gray).