Membrane and vesicular trafficking in the early secretory pathway are mediated by non-clathrin COP (coat protein) I-coated vesicles. COPI-coated vesicles mediate retrograde transport from the Golgi back to the ER and intra-Golgi transport. The cytosolic precursor of the COPI coat, the heptameric coatomer complex, is composed of two subcomplexes. The first consists of the COPB, COPG, COPD and COPZ subunits (also known as b-, g-, d- and z-COP, respectively), which are distantly homologous to AP Clathrin adaptor subunits. The second consists of the COPA, b'-COP and COPE subunits (also known as a-COP, COPP and e-COP, r espectively). The COPG (g-COP) subunit of the coatomer is believed to mediate the binding to the cytoplasmic dilysine motifs of membrane proteins. COPG has the highest level of expression in mouse testis and is expressed in a parent-of-origin-specific manner in mammals.