A method useful for identifying and isolating live circulating tumor cells is described. The method utilizes an adenoviral vector comprising a replication-competent adenovirus in which the E1 gene region is expressed under the control of a telomerase-specific promoter and further comprises a second expression cassette containing a marker protein, optionally fused to a detectable cell surface marker to permit detection of circulating tumor cells lacking cell surface markers. The method involves combining ex vivo a test sample from a patient suspected of having circulating tumor cells, an adenoviral probe system, and culture media for the cells. The test sample is incubated with the adenoviral system for a sufficient time to permit expression of the reporter protein. The marker gene expression can thereafter be quantitated and the marker-expressing cells may optionally be collected for further analysis.