Simian virus (SV40) large T antigen, molecular model. This antigen is from the simian vacuolating virus 40 (SV40). Large T antigens play a role in regulating the viral life cycle of the polyomaviridae viruses, such as SV40. SV40 is found in monkeys such as Rhesus monkeys and macaques. Potentially tumour-causing in primates and humans, it is used in laboratory research and in vaccines. The structure of this antigen is a circular hexamer (6-fold structure), formed by two entwined protein chains (red and yellow).