PRF1 and neoplasm: Upon antigen-specific activation, CTLs proliferate and differentiate into two major subsets: effector CD8+ T cells, characterized by high expression of granzyme, perforin, and IFN-γ, which eliminate target tumor cells; and memory CD8+ T cells that possess self-renewal and multilineage differentiation capacities, providing a cellular reservoir for long-term immune surveillance (4, 5).