However, the GEN2 directed cancer immunotherapy trial is the only one that includes monitoring of HSV-1-tk-m2 expression using [18F]FHBG PET imaging (NCT04313868) while its use to monitor HSV-1-tk gene therapy in humans has already been validated [105], including the use of [124I]FIAU as the reporter probe [92]. Here, TKT is linked to cancer.