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Helicobacter pylori virulence factor CagA (cytotoxin-associated gene A) is a 120–145kDa protein encoded on the 40kb cag pathogenicity island (PAI). H. pylori strains can be divided into CagA positive or negative strains. Around 60% of H. pylori isolates in Western countries are positive, whereas the majority of East Asian isolates are positive.

The cag PAI also encodes for a type 4 secretion system which is used to "inject" CagA into a target cell upon H. pylori attachment. After translocation, CagA localises to the inner surface of the cell membrane and undergoes tyrosine phosphorylation by Src family kinases (e.g. Fyn and Lyn).