Both natural viral infections and therapeutic interventions using viral vectors pose significant risks of malignant transformation. Monitoring for clonal expansion of infected cells is important for detecting cancer. Here we developed a novel method of tracking transgene integration sites. RAISING (Rapid Amplification of Integration Sites without Interference by Genomic DNA contamination) is a sensitive, inexpensive alternative to established methods. Its compatibility with Sanger sequencing combined with our CLOVA (Clonality Value) software is critical for those without access to expensive next-generation sequencing. To model our method, we analyzed samples from 698 patients infected with the retrovirus HTLV-1, which causes adult T-cell leukemia/lymphoma (ATL). We defined a clonality value identifying ATL patients with 100% sensitivity and 95.3% specificity, and our preliminary longitudinal analysis suggests this may also be useful for ATL risk assessment. We anticipate future studies will confirm the broad applicability of our technology, especially in the emerging gene therapy sector.