Epistasis in RNA Viruses

Several recent papers document epistasis in RNA viruses. See, for example:

Sanjuan R, Moya A, Elena SF. The contribution of epistasis to the architecture of fitness in an RNA virus. Proc Natl Acad Sci U S A. 2004 Oct 26;101(43):15376-9. [PubMed]

Bonhoeffer S, Chappey C, Parkin NT, Whitcomb JM, Petropoulos CJ. Evidence for positive epistasis in HIV-1. Science. 2004 Nov 26;306(5701):1547-50. [PubMed]