 Human Parainfluenza Virus type 1 (HPIV1), which is a close relative of SeV and is (in contrast to SeV) a successful human pathogen, does not express V proteins, only C proteins.So, all needed functions provided by V in SeV can be provided by C in HPIV1.Thus, C and V have these "overlapping functions" because of the multi-faceted nature of host defense that can be countered at so many places, and exactly how well and where will in part explain host restriction.