Poliovirus, like other positive-stranded RNA viruses, replicates its RNA genome using a polymerase that associates with intracellular membranes. Several of the proteins encoded in its genome alter the structure or dynamic behavior of the membrane-enclosed organelles in the host cell. These electron micrographs show a normal Cos-7 cell (left) and a cell expressing the 3A protein from poliovirus (right). In the transfected cell, the ER is swollen and traffic from the ER to the Golgi is inhibited. (From J.J.R. Doedens, T.H. Giddings Jr., and K. Kirkegaard, J. Virol. 71:9054–9064, 1997.)