SV40 Large T Antigen is the major regulatory protein of SV40 virus, responsible for virion assembly, viral and cellular transcriptional regulation, viral DNA replication, alteration of the cell cycle. It also functions as molecular chaperone. SV40 Large T Antigen plays multifunctional roles: functions as an initiator protein to begin DNA replication at the SV40 origin, performs as a DNA helicase that unwinds DNA ahead of the polymerizing complex, acts as a primosome loading protein, exhibits ATPase activity. Simian virus (SV40) DNA is the best model for mammalian chromosomal DNA replication in vitro.