The large tumor antigen (also called the large T-antigen and abbreviated LTag or LT) is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. LTag is expressed early in the infectious cycle and is essential for viral proliferation. Containing four well-conserved protein domains as well as several intrinsically disordered regions, LTag is a fairly large multifunctional protein; in most polyomaviruses, it ranges from around 600-800 amino acids in length. LTag has two primary functions, both related to replication of the viral genome: it unwinds the virus's DNA to prepare it for replication, and it interacts with proteins in the host cell to dysregulate the cell cycle so that the host's DNA replication machinery can be used to replicate the virus's g