The exact location of the ribosomal frameshift is unknown. The F protein seems to be generated by a -2 ribosomal frameshift located in the vicinity of codon 11 of the core protein coding sequence. However, some F proteins may also be generated by +1 ribosomal frameshift. Since the core gene encodes alternative reading frame proteins (ARFPs), many functions depicted for the core protein might belong to the ARFPs. This protein is very unstable. Produced by ribosomal frameshifting.