Since it was discovered, twenty years ago, the glial fibrillary acidic protein has been the subject of more than 500 publications. The huge interest it has raised up is probably due partly to its abundance in the central nervous system and also, above all, to its cellular specificity which makes it the universally recognized marker of astrocytes. It has been used by biologists as a tool to follow the normal or pathological glia cell differentiation in animal models and human pathology, particularly in the fields of neuropathology and neuro-oncology.