Catalase (CAT) is a member of the catalase family. It exists as a homotetramer that occurs in almost all aerobically respiring organisms and serves to protect cells from the toxic effects of hydrogen peroxide. Catalase is localized in the peroxisome. Catalase promotes growth of cells including T-cells, B-cells, myeloid leukemia cells, melanoma cells, mastocytoma cells, and normal and transformed fibroblast cells. Defects in CAT are the cause of acatalasemia which is characterized by absence of catalase activity in red cells and is associated with ulcerating oral lesions.