Regulates cytotoxic granule exocytosis in effector lymphocytes, thus acting as a critical mediator of inflammation in a broad range of infectious and non-infectious diseases (By similarity). Essential for cytotoxic degranulation of natural killer (NK) cells and CD8(+) T-cells and for the activation of CD4(+) T-cells following infection (By similarity). Plays a critical role in CD8(+) T-cell and NK cell-mediated cytolysis of target cells and contributes to the cytolytic activity via the perforin/granzyme pathway by enhancing exocytosis of LAMP1-carrying lytic granules (By similarity). Contributes to NK cell-mediated control of cancer metastasis (By similarity). Belongs to the PMP-22/EMP/MP20 family.