Within the cag PAI region there are 31 open reading frames ORF. One of these ORFs encodes the immunodominant antigen CagA which is located towards the 3 end of the pathogenicity island. CagA protein was first identified in PAI and is the most important of H. pyloris virulence factors. Its presence cagA + is a direct marker for PAI. The protein was discovered in the early 1990s independently by Cover et. al. 15 Jean Crabtree et. al.
Dentro de la región cag PAI hay 31 marcos abiertos de lectura ORF, uno de estos ORF codifica para el antígeno inmunodominante CagA, que está localizado hacia la región 3´ final del islote de patogenicidad; CagA fue la primera proteína identificada de PAI y el mayor factor de virulencia de H. pylori, por lo que su presencia cepas cagA + es un marcador directo de PAI.