The localization of each processing step shown was determined by a combination of techniques, including biochemical subfractionation of the Golgi apparatus membranes and electron microscopy after staining with antibodies specific for some of the processing enzymes. The locations of many other processing reactions have not been determined. Although only three distinguishable cisternal compartments have so far been demonstrated, each of these sometimes consists of a group of two or more cisternae in sequence. It is likely that each processing enzyme is not completely restricted to a particular cisterna but that its distribution is graded across the stack—such that early acting enzymes are present mostly in the cis Golgi cisternae and later acting enzymes are mostly in the trans Golgi cisternae.