(TNFR1) and TRAIL receptor (TRAILR)1–2, respectively).28
Alternatively, an extrinsic pro-apoptotic signal can be dispatched
by the so-called ‘dependence receptors’, including
netrin receptors (e.g., UNC5A-D and deleted in colorectal
carcinoma, DCC), which only exert lethal functions when the
concentration of their specific ligands falls below a critical
threshold level.30