polyomavirus genome. MCPyV possesses four early proteins, including a 57kT alternative splicing isoform and an alternate protein called ALTO. In Merkel cell polyomavirus, unlike in SV40, LTag alone does not support efficient viral replication and STag is required. Comparison of the sequences of MCPyV and SV40 LTag predicts that they have similar capacities for protein-protein interactions, including preservation of the Rb and p53 binding sites. Mutations in MCPyV LTag associated with tumors consist of large C-terminal truncations that eliminate the DNA replication functions of the protein by removing the zinc-binding and ATPase/helicase domains, without affecting these protein-protein interaction sites. Role in taxonomy LTag is a large protein whose domains can be detected and annotated bioinformatically. As a result, it is often used in comparing and determining relationships among polyomaviruses. The International Committee on Taxonomy of Viruses currently classifies polyomaviruses primarily according to the sequence identity of their LTag genes. This system has been questioned by phylogenetic studies suggesting that the evolutionary histories of LTag and major capsid protein VP1 are divergent and that some modern polyomavirus represent chimeric lineages. References Viral oncoproteins Polyomavirus proteins