amino acids, whereas Rep' is a truncated form of ORF1 as a result of splicing and is only 168 amino acids in length. The promoter for rep (Prep) contains an Interferon-Stimulated Response Element (ISRE) that suggests Rep and Rep' are regulated by cytokine involvement, and is probably a means for the virus to overcome the host's immune responses to infection. Rep and Rep' form a dimer that binds to two hexameric regions adjacent to the stem-loop, H1 and H2, which is required for replication. When the dimer binds to this region, the replicases cleave the loop region of the stem-loop and remain covalently bound to the H1 and H2 regions of the DNA, which becomes the 5' end of the DNA. The newly formed 3'OH end forms a primer using host RNA polymerase, which is then used by the host's DNA polymerase to begin transcription of the viral DNA via rolling circle replication. After the complementary DNA strand has been created, the stem region of the stem-loop forms a loose, non-hydrogen bonded, quadruplet DNA structure. This loosely associated structure can form short lived DNA-trimers which forms two templates for replication, as well as maintaining the nucleic integrity of the stem region of the stem-loop. The termination of the replication sequence has not been identified, yet, though there is evidence supporting that Rep also represses its own promoter, Prep. The ORF2 region encodes the capside protein Cap (aka CP), which differs slightly between PCV-1 and PCV-2. This variation within PCV may explain why PCV-1 is non-pathogenic, while PCV-2 is pathogenic. The promoter for this protein is located within ORF1, within the site where Rep' is truncated, and is splice from the same exon to the starting point of the ORF2 coding region and expressed during both early and late phases. This is the immunogenic region of the virus and is the primary area of research for creating a vaccine to treat PMWS. There is a third gene encoded in the opposite orientation to ORF1 in the genome. This gene is transcribed and is an essential gene involved in viral replication. Size Porcine circovirus is a replicating entity with one of the smallest DNA strands consisting of a simple loop of DNA. The DNA sequence for Porcine circovirus type 2 strain MLP-22 is 1726 base pairs long. Entry PCV infects a wide variety of cell types, including hepatocytes, cardiomyocytes, and macrophages. However,