Conventional and unconventional secretory pathways. In conventional secretion (left), cargoes with a signal peptide enter the ER with the signal peptide removed. The secretory cargoes are then delivered out of the cell through vesicle trafficking via the ERGIC and Golgi. In unconventional secretion (right), four types of pathways have been classified. The leaderless cargoes can directly translocate across the plasma membrane (type I) or via ABC transporters (type II). The cargoes can also enter into a membrane-bound organelle which ultimately reaches the plasma membrane through membrane trafficking (type III). In addition, the transport of transmembrane proteins towards the plasma membrane or post-Golgi compartments can occur independently of the Golgi (type IV).