Absolute CD3⁺ T-cell counts (including CD3⁺CD56⁺) and numbers of suicide gene-modified (SGM) donor T lymphocytes during the first 100 days after PBSCT in all 3 patients treated according to our protocol. Patient 2 showed stable numbers of SGM cells for about 3 months accompanied by increasing absolute CD3⁺ counts and full donor chimerism. In contrast, in both patients 1 and 3 early in vivo depletion of SGM donor T lymphocytes was observed, mediated by ganciclovir applied to treat an acute GvHD grade II (patient 3) or most probably as the result of an anti-HSV-thymidine kinase (tk) immune reaction (patient 1). Both patients appeared to have higher absolute CD3⁺ counts on day 100 compared with patient 2, but developed mixed chimerism (not shown) and eventually rejected their grafts at days 156 and 119. Arrows indicate a second donor SGM T lymphocyte infusion in patient 1 (day 65) and patient 2 (day 58). Note that different Y-axes should be applied to CD3⁺ and SGM cells.