TWO MAJOR PATHWAYS. “The extrinsic and intrinsic pathways represent the two major well-studied apoptotic processes. The extrinsic pathway is mediated by a sub-group of Tumor Necrosis Factor receptors (TNFR) superfamily that includes TNFR, Fas and TRAIL.”
http://www.sciencedirect.com/science/article/pii/S0167488910002764