During cytomegalovirus (CMV) infection, gene expression is regulated by transcriptional and posttranscriptional events. Although recent studies have established that posttranscriptional controls are important determinants of gene expression in several eukaryotic systems, the precise signals and mechanisms have not been clearly identified. We present evidence for a cis-acting signal, contained within the 5' leader region of a CMV beta (or early) gene, that acts posttranscriptionally in the regulation of gene expression. Addition of this signal to an alpha (or immediate early) gene construct converted expression of the indicator protein to the beta class, even though the gene remained under alpha transcriptional control. Deletion of a portion of the beta gene leader sequence (nucleotides +62 to +142) reverted expression to the alpha class. This cis-dominant signal appears to act by blocking expression posttranscriptionally until a viral function activates full gene expression at the appropriate time in infection.