Periodic acid Schiff (also known as PAS) is a special stain test performed by pathologists to look for two types of chemicals in a tissue sample: a type of sugar called glycogen, and a type of protein called mucin. Pathologists often perform a PAS stain when they are trying to determine if the cells in the tissue sample are producing glycogen or mucin. Pathologists also sometimes use PAS to look for fungal micro-organisms such as Candida or Pneumocystis. When a PAS-stained slide is examined under the microscope, both glycogen and mucin appear bright pink or red. Fungal micro-organisms will also appear bright pink or red. This test is often performed at the same time as a periodic acid Schiff plus diastase (PAS-D) stained slide. PAS-D is very similar to PAS with the exception of an added chemical called diastase. Diastase breaks down glycogen which makes it easier to see mucin.