Which of the following feature is common to both cytotoxic T-cells and NK cells?

Cytotoxic T lymphocytes plays a protective role against virus associated neoplasms (EBV and HPV induced tumors) and have been demonstrated in the blood and tumor infiltrates of cancer patients.NK cells have the innate ability to lyse a variety of tumor cells, virally infected cells and some normal cells without prior sensitization. It is considered to be the first line of defence against neoplasm or viral infections. Ref: Robbins Pathologic Basis of Disease, 6th Edition, Page 191 and 8th Edition, Page 318