Rockville, Md --GenVec has begun a phase I study of its in vivo gene-based product, AdgvCD. 10, in 18 patients with liver metastases of colectoral cancer . An advenovirus vector modified to carry the cytosine deaminase gene will be administered directly into the hepatic tumor. The gene expresses an enzyme that converts 5-fluorocytosine,an oral anti-fungal agent , into the potent anticancer agent 5-fluorouracil.. The conversion occurs only at the site of gene expression, the company said.