Tumours develop resistance to CTL-mediated killing mechanisms by inhibiting the perforin/granzyme pathway through expression of granzyme-specific serine proteases (serpins); and may express decoy receptors, e.g., soluble Fas and decoy receptors 3 and 4 (DcR3, DcR4) for the death receptors Fas and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) [17]. Here, PRF1 is linked to neoplasm.