Periodic Acid-Schiff Stain (PAS) is a histological staining technique used to detect polysaccharides, such as glycogen and mucosubstances in tissues. The stain works by oxidizing the carbohydrates, which then react with the Schiff reagent to produce a bright magenta color. This makes it particularly useful in identifying certain types of cells and tissues, including those in fungi and certain tumors. PAS is commonly employed in pathology to diagnose conditions like diabetes mellitus and mucopolysaccharidoses. It can also help visualize structures like basement membranes and glycoproteins, providing valuable information about tissue composition and disease states.