Simian virus 40 is a small DNA virus encoded by 5.2 kb of double-stranded DNA. SV 40 large T antigen (T-ag) is a multifunctional 85 kD phosphoprotein, which is the sole viral protein required for SV40 replication. All other factors are provided by the infected host cell. In addition to its role in SV40 DNA replication, T-ag also causes transformation of susceptible cell lines. Studies of various mutant T-ag proteins have shown that the replication and transformation fractions of T-ag can be separated. The multifunctional nature of this protein has resulted in its use as a model system in a wide variety of disciplines. SV40 T-ag exercises negative regulation on the transcription of SV40 early mRNA by feedback inhibition and exerts positive regulation on transcription from the late promoter. In addition to transcriptional regulation, T-ag is involved in viral DNA replication. Specific biochemical functions required for DNA synthesis that are inherent to the T-ag include high-affinity binding to sites within the viral origin of DNA synthesis and ATPase and helicase activities. Other functions attributed to T-ag include cellular transformation, induction of cellular DNA synthesis, induction of rRNA synthesis, and provision of a host-range function for viral replication. However, all functions of T-ag are influenced by a wide range of post-translational modifications including phosphorylation, glycosylation, acetylation, acylation, and adenylation. T-ag exists in monomeric as well as polymeric forms and associates with the tumor suppressor proteins p53 and Rb (retinoblastoma protein). Most of T-ag is transported to the nucleus, while a small fraction is localized at the cell surface. PAb 100 recognizes an epitope between amino acids 270 and 520 of T-ag. PAb 100 was originally known as clone 412. Studies have suggested that PAb 100 binds the strongest to newly synthesized T-ag and to ATPase-active T-ag in some experimental systems. In other experimental systems it appears to preferentially bind to recognize mature T-ag. PAb 100 (i.e., clone 412) was developed along with a panel of monoclonal antibodies where SV40-transformed BALB/c mouse cell lines (SVT2 or B4) were used as immunogens. The specificity of the antibody was originally characterized by a variety of techniques using SV40-infected and SV40-transformed cells.