In our previous phase 1/2 study aimed at controlling graft-versus-host disease, 12 patients received Herpes simplex virus thymidine kinase (HSV-tk ؉ )/ neomycin phosphotransferase (NeoR ؉ )-expressing donor gene-modified T cells (GMCs) and underwent an HLA-identical sibling T-cell-depleted bone marrow transplantation (BMT). This study's objective was to follow up, to quantify, and to characterize persistently circulating GMCs more than 10 years after BMT. Circulating GMCs remain detectable in all 4 evaluable patients. However, NeoRand HSV-tk-polymerase chain reaction