Periodic acid–Schiff (PAS) is a staining method used to spot polysaccharides such as glycogen, and mucosubstances such as glycoproteins, glycolipids, and mucins in tissues. The reaction of periodic acid oxidizes the vicinal diols in these sugars, generally breaking up the bond among two nearby carbons not tangled in the glycosidic linkage or ring closure in the ring of the monosaccharide units that are parts of the stretched polysaccharides, and forming a pair of aldehydes at the two free tips of each cracked monosaccharide ring. The oxidation condition has to be appropriately regulated so as to not oxidize the aldehydes more. These aldehydes then respond with the Schiff reagent to give a purple-magenta color. A suitable basic stain is frequently used as a counterstain. PAS diastase stain (PAS-D) is PAS stain used in amalgamation with diastase, an enzyme that cessations down glycogen. Alcian blue/periodic acid–Schiff (AB/PAS or AB-PAS) uses alcian blue before the PAS step.