Human endogenous retroviruses (HERVs) comprise a significant portion of the human genome. They are expressed in a variety of human tissues and some encode intact retroviral proteins. HERV expression seems to be upregulated in certain tissues and tumour types. The HERV-K(HML-2) family is an important exception from the vast majority of defective HERVs in that it encodes all essential retroviral proteins and functional enzymes, respectively. Furthermore, several intact HERV-K(HML-2) gag and env genes as well as an almost intact HERV-K(HML-2) provirus could be demonstrated within the human genome. However, none of the HERVs described so far seems to produce infectious virus.