Regulation of nuclear export by the HIV Rev protein. (a) Early in HIV infection, only fully spliced viral mRNAs, containing the coding sequences for Rev and other proteins, are exported from the host cell nucleus and translated. (b) Once sufficient Rev protein has accumulated and been transported into the nucleus, unspliced viral RNAs can be exported into the cytoplasm. Many of these RNAs are translated into proteins. These and all other viral proteins are packaged into new viral particles.