Answer: HIV & related viruses such as HTLV possess unusual virally encoded reverse transcriptase enzyme.
Reverse transcriptase is an enzyme which synthesizes a complementary DNA or cDNA from a RNA template strand. The process of reverse transcription is used by the retroviruses for replicating their genome and proliferating within the host genome. The HIV uses reverse transcriptase to infect humans. Without the reverse transcriptase enzyme the genome of the virus cannot incorporate into the host cell and will fail to replicate. HTLV or human T-lymphotropic virus or HTLV also uses the same mechanism with the enzyme reverse transcriptase.