The rationale of the suicide gene approach is that cells transduced with the construct encoding a hygromycin phosphotransferase (Hy) thymidine kinase (TK) fusion gene can be selected on the basis of hygromycin resistance and then eradicated if adverse effects occur by administration of gangiclovir which will be converted by Tk to a cytotoxic agent (left). However, the viral-derived Tk product and bacterial-derived Hy products proved immunogenic presenting multiple peptides that could generate a cytotoxic T cell response that effectively eradicated transduced cells (right). Illustration by Frank Forney.