It has been shown that SPECT imaging with 123I-2′-fluoro-2′-deoxy-5-iodo-1-β-D-arabinofuranosyluracil (123I-FIAU) is a reliable method for predicting tumor response to GCV treatment, which was found to be proportional to the magnitude of HSV1-tk expression in tumor tissue [9]. This evidence concerns the gene TKT and neoplasm.