Transgene-specific effectors are present at low frequency in a healthy donor. PBMCs containing 2 × 10⁶ CD3⁺ lymphocytes isolated from the donor of patient 2 were stimulated, in 20 independent microcultures, with autologous GMCs. On day 10, lytic activities of the lymphocytes were measured against SFCMM2-transduced (○) and untransduced (●) autologous GMCs as well as allogeneic T cells (□) in a standard chromium-release assay at the indicated E/T ratios. As shown, an immune response specific for the transduced cells was detected in microculture 6.