SV40, or Simian Virus 40, is a polyomavirus that was discovered in the 1960s. It naturally infects monkeys, but it can also infect humans and other mammals in laboratory settings. The virus has a small, circular double-stranded DNA genome that encodes several genes. The most well-known and studied SV40 genes are the large T antigen (Tag) and the small t antigen (tag).

1. Large T antigen (Tag): The large T antigen is a multifunctional protein that plays a critical role in viral replication, regulation of host cell cycle, and transformation of host cells. It binds to the viral origin of replication (ori) and assists in the initiation of DNA replication. Moreover, it has the ability to interact with several cellular proteins, such as retinoblastoma protein (Rb) and the tumor suppressor protein p53, disrupting their normal function and leading to cell transformation.
2. Small t antigen (tag): The small t antigen is a less well-characterized protein compared to the large T antigen. It has been shown to have transforming activity in some cell types, and it can cooperate with the large T antigen in transforming cells. The small t antigen is also involved in the regulation of protein phosphatase 2A (PP2A), a cellular enzyme that plays a role in various cellular processes.

Besides these two main genes, SV40 also encodes for other proteins, such as VP1, VP2, and VP3, which are the viral capsid proteins, and agnoprotein, a regulatory protein involved in viral replication and assembly.

Research on SV40 has contributed significantly to our understanding of viral replication, cellular transformation, and the molecular basis of cancer. However, the role of SV40 in human diseases, particularly cancer, is still a topic of ongoing debate and research.