Although the herpesvirus genome has been completely sequenced, the roles of many gene products remain unknown. Unlocking these secrets demands strategies to mutate individual genes and see how this affects viral life cycle. To make this easier, Brune et al. have developed a rapid mutagenesis strategy to insert transposons randomly into the herpesvirus genome cloned in bacterial artificial chromosomes (BACs) (see pp. 332 and 360). This method, which should be applicable to any cloned virus, can be used to distinguish rapidly between genes that are essential or nonessential for viral replication.