PPIA and viral infectious disease: It has been demonstrated that the PPIase activity can catalyze isomerization of the peptide bond upstream of proline residues in proteins and it is required for protein folding, protein trafficking/molecular chaperoning, cell signaling and T cell activation.67 For instance, CypA can catalyze the prolyl cis-trans isomerization of proteins like interleukin-2 tyrosine kinase and cell signaling adaptor protein Crk, influencing their functions.71–73 In addition, the interaction between CypA and viral proteins plays an important role in the process of viral infection.