Virology![Figure][1] Transmission electron micrograph of a Mollivirus particle PHOTO: M. LEGENDRE ET AL., PNAS PLUS (8 SEPTEMBER 2015) © PNAS A recent fascinating development in basic virology has been the discovery of “giant” viruses that are visible by light microscopy. Legendre et al. now report a fourth type of giant virus called Mollivirus sibericum . Like its cousin Pithovirus sibericum , it can still infect acanthamoeba (a common soil protozoan) after being found in 30,000-year-old Siberian permafrost. Its diameter spans 0.6 µm, with a 623-kb genome, but it differs from other giant viruses in how it replicates, how its genome is organized, and in the proteins it encodes. Nearly 65% of the proteins encoded by Mollivirus have no known homologs. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1510795112 (2015). [1]: pending:yes