FURIN and infection: Although we could not detect any obvious morphological differences between uncleaved and FPC HPV16-PsVs by negative electron microscopic (EM) staining (Fig. 1B), furin cleavage had a substantial functional impact on infection of the HSPG-deficient cell line pgsD677: while pgsD677 cells were practically noninfectible by HPV16-PsVs, furin cleavage of the particles led to an approximately 40-fold increase in infection as measured by luciferase reporter gene activity (Fig. 1C).