Human papillomaviruses type 16 (HPV-16) is one of a number of HPV types specifically associated with the development of carcinoma in the anogenital tract and HPV-16 is the most commonly found subtype in cervical carcinomas worldwide. The early HPV proteins are expressed throughout epithelial differentiation, but expression of the late proteins is tightly linked to the differentiation status of the host cell, being found only in cells in the process of keratinisation in the granular layer of stratified epithelia. For HPV-16, a 79 nt negative regulatory element (NRE), overlapping the 3' end of the LI stop codon and extending into the late 3' untranslated region, is an important post-transcriptional regulator of viral late gene expression. Unsurprisingly, the HPV-16 NRE has previously been shown to bind a range of RNA processing factors in vitro, including the auxiliary splicing factor U2AF65, the polyadenylation factor CstF-64, and the elav-Wke HuR protein. I carried out saturating site-directed mutagenesis of the HPV-16 NRE and found that no individual short sequences are responsible for the activity of this inhibitory element, or its ability to interact with cellular factors. I have used an affinity-chromatography based RNA binding assay to show that the NRE can also bind the splicing-related (SR) protein, ASF/SF2, a member of the Sm protein family, and the U1A subunit of UlsnRNP. Furthermore, using a novel monolayer culture differentiation system and organotypic rafts to generate undifferentiated and differentiating populations of HPV-16 positive epithelial cell lines, I have shown by Western blot analysis that the level of expression of the NRE-binding proteins is differentially regulated, and is influenced by the status of the viral genome in the infected cell. I suggest that the inhibitory activity of the HPV-16 NRE is overcome as a result of the upregulation of cellular factors that bind the NRE.