- Retroviruses contain positive-sense single-stranded RNA (+ssRNA) as their genetic material.
- This allows their RNA to serve directly as a template for reverse transcription, producing complementary DNA (cDNA) that integrates into the host genome.
- Unlike DNA viruses or negative-sense RNA viruses, retroviruses uniquely use reverse transcriptase enzyme to convert their +ssRNA into DNA, which is essential for replication.

Hence, +ssRNA is the defining feature of retroviruses.

Reference: Medical Microbiology, 8th Edition, Chapter 33, Retroviruses