When virus infection occurs, the expression of MHC class I molecules on the surface of virus-infected cells is deleted or down-regulated, while some non-MHC Class I molecular ligands that can be recognized by NK cells on the surface are abnormal or up-regulated, and NK cells are further activated to kill the target cells of virus infection by releasing perforin, granase, TNF-α and expressing FasL (Cao, 2017). This evidence concerns the gene FASLG and viral infectious disease.