APEX is an engineered peroxidase that can function as an electron microscopy tag. It is genetically fused to a protein of interest, and the fusion construct is expressed inside cells. APEX catalyzes the polymerization and local deposition of diaminobenzidine which is detected in electron microscope. This technique is applicable to many cell types, intact tissues and organisms, and is useful for a variety of applications.
Possible applications of labeling with APEX:
Some examples can be found here, here and here.