Perforin (Pore-forming protein) is originally cloned from mouse cytotoxic T-lymphocyte (CTL). It is a 555-amino acid protein with a 21-amino acid signal peptide. Perforin is a cytolytic mediator produced by killer lymphocytes, and is stored in and released by cytoplasmic granules. Perforin is involved in the killing mediated by CTL which represents an important mechanism in the immune defense against tumors and virus infection.