Retroviruses are a group of single-stranded RNA viruses with the ability to integrate their genome into the host-cell DNA.

Upon infection into a suitable host cell, the retroviral core enters the host-cytoplasm. The core contains the viral-genetic material and viral-enzymes like reverse transcriptase and integrase.

Once inside the host-cytoplasm, the core disassembles to release its contents. Then, the viral reverse transcriptase enzyme reverse-transcribes the viral-RNA into single-stranded complementary DNA or cDNA.

The cDNA replicates to generate double-stranded DNA that enters the host nucleus along with the viral-integrase enzyme. The integrase enzyme catalyzes the incorporation of the double-stranded viral-DNA, called provirus into the host-DNA.

The integration of the provirus into the host genome can activate or transform a normal cell into a cancerous cell via several different mechanisms. Such retroviruses are called cancer-causing or oncogenic viruses.

For example, the promoter and enhancer sequences present in the provirus may activate the neighboring host genes and trigger abnormal cell growth.

Expression of some viral proteins inside the host cell may also indirectly activate the host genes and result in their aberrant expression.

At times, the integrated provirus may contain oncogenes in addition to the normal retroviral genes, which upon expression in the host cell drive tumor formation.