The classification of viral proteins as early proteins or late proteins depends on their relationship with genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 replication. While many viruses (such as HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

) 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 nucleus.)

Early proteins are those produced following entry

Viral entry

Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Despite the variation among viruses, the generalities are quite similar...

 into the host cell but prior to replication

Viral replication

Viral replication is the term used by virologists to describe the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. From the perspective of the virus, the purpose of viral replication is...

. The expression of these genes, commonly encoding non-structural proteins, initiates replication of the genome and expression of late genes.

In some, simpler viruses, this pattern of expression is clearly defined, while in those with more complex genomes, such as the herpesviruses, these expression periods overlap.

Example

An example of early gene expression is the expression of the small, middle and large T antigen encoded by the polyomavirus. The middle T antigen isn't required for replication and it acts to enhance transcription by binding host proteins which interact with the late promotor. On the other hand the large T antigen is required and it acts to initiate replication directly. It binds the viral origin of replication and recruits DNA polymerase and s/s DNA binding protein such that once its concentration is great enough it blocks the transcription of early genes and initiates genome replication. It also acts to cause the entry of the host cell into S phase.