Fas ligand is a type II transmembrane protein that belongs to the tumor necrosis factor (TNF) family. The binding of Fas ligand with its receptor induces apoptosis. Fas ligand-receptor interactions play an important role in the regulation of the immune system and the progression of cancer.Fas ligand or FasL is a type II transmembrane protein.

Fas ligand is a homotrimeric protein and signals through trimerization of FasR, which spans the membrane of the "target" cell. This trimerization usually leads to apoptosis, or cell death.Soluble Fas ligand is generated by cleaving membrane-bound FasL at a conserved cleavage site by the external serine matrix metalloproteinase MMP-7. Soluble FasL is less active than their membrane-bound counterparts and do not induce receptor trimerization and DISC formation.