There are three known mechanisms for inactivating the Notch pathway in SCLC: (1) the mutational inactivation of Notch pathway genes; (2) the inhibition of Notch receptors by canonical Notch ligand Delta-like ligand 3 (DLL3) or Delta-like non-canonical Notch ligand 1 (DLK1); and (3) the degradation of Notch receptors guided by DLL3 in the endosomes [42]. Here, DLK1 is linked to small cell lung carcinoma.