Perforin is a cytolytic protein found in the granules of CD8 T-cells and NK cells. Upon degranulation, perforin inserts itself into the target cell's plasma membrane, forming a pore. The lytic membrane inserting part of perforin is the MACPF domain. This region shares homology with cholesterol dependent cytolysins from Gram positive bacteria.

Although purified perforin is sufficient to lyse cells at high doses, the biology of perforin itself does not explain the ability of CD8 T-cells and NK cells to induce apoptosis in target cells. This induction of apoptosis may require at least one other granule protein, granzyme B.