The primary objective of this study was to verify that UL21.5 mRNA detected in plasma is actually included into viral particles and could represent an indirect marker for active replication. For this aim, retrospectively collected plasma samples from immunocompromised patients were tested for HCMV UL21.5 mRNA before and after ribonuclease (RNAse) digestion (RNAse protection assay) to distinguish between virion-packaged and free-floating naked RNA.