The rate of adenovirus replication depends on the serotype of virus, the cell type, and the m.o.i. For the prototype strain Ad5, infection of HeLa cells with a modest m.o.i. of about five infectious particles per cell results in detectable viral early gene expression by 4 h postinfection, detectable DNA replication by 10 h postinfection, production of structural proteins that will form progeny virions by about 18 h postinfection, and release of infectious particles from about 30 h onward. To ensure the isolation of intact nucleoli, we chose to examine the composition of the nucleoli at ϳ18 h postinfection when late structural gene expression would be just underway in some cells but when active viral replication would be well underway in virtually all the cells examined (for a review, see Ref. 49) .