Two different types of cell cultures were used for virus strain propagation: diploid HELF (human embryonic lung fibroblast) and a cell line of human retinal pigmented epithelial cells (ARPE-19, ATCC CRL-2302, Manassas, VA, USA). HELF were derived from a cell strain developed in our laboratory in 1980 and used at passages 20–30. AD169 a highly passaged laboratory-adapted strain was inoculated onto HELF cells and the HCMV isolate VR1814, which was originally recovered from cervical secretions, onto ARPE-19 cells until a cell-free virus stock with a sufficient infectious titer (2x106 and 4x106, respectively) was obtained in the cell-culture supernatant. The infected cell supernatant was then filtered with a sterile syringe using 0.45 µm filter (VWR International Srl, Radnor, PA, USA) in order to eliminate cellular debris. UL21.5 mRNA quantification and RNAse protection assay were performed as described above.