The classification of viral proteins as early proteins or late proteins depends on their relationship with genome replication. While many viruses (such as HIV) are described as expressing early and late proteins, this definition of these terms is commonly reserved for class I DNA viruses. (HIV has two stages of protein expression but these are not as a result of two stages of transcription surrounding replication but by the production of the Rev protein which is required for the export of the transcripts of the second set of proteins transcribed form the cell nucleus.)