This is some research from Dec 2000, that I've just come across, which is suggesting that breast cancer might be caused, in part, by a human retrovirus; the 'human homologue of the mouse mammary tumour virus' (HHMMTV). http://breast-cancer-research.com/content/3/2/81 Am I missing something here? I don't totally understand it... Are they saying that HHMMTV is primarily an endogenous or an exogenous virus? And if it is both, then what are the implications? I don't totally understand it because it looks like HHMMTV might be both endogenous and exogenous... So is it another HERV (human endogenous retrovirus) that expresses itself fully to form an exogenous virus, which I didn't think was possible for HERV's, or is it a new exogenous human retrovirus? Has anyone got any insight into this? A lot of research seems to end up quietly disappearing after it can't be replicated, so maybe it is one of those studies... I haven't looked up any further research studies on the subject yet.