Human retrovirus. Coloured transmission electron micrograph (TEM) of HERV-K virus particles (green and red). HERV-K is human endogenous retrovirus type-K. Retroviruses are RNA viruses that copy their genome into the DNA of the host cell. This can then lie dormant and be passed on through the generations, when it is known as an endogenous retrovirus. If it then reactivates, it may cause diseases such as cancer, though the link is not yet certain. HERV-K is associated with male germ cell tumours (usually testicular cancer).