CLOCK and infection: Thus, it has been proposed that the M protein may function as a type of ‘biological clock’ for the virus by entering the nucleus early in infection to inhibit host cell transcription, thereby focusing the efforts of the infected cells machinery on viral replication, then exiting late in infection from the nucleus to terminate viral replication in preparation for virus assembly, only once sufficient viral protein has accumulated in cells [2, 6, 7, 8].