In such cases (e.g., Zellwegers Syndrome, and other peroxisomal deficiency diseases), peroxisomal matrix proteins such as catalase and ACOX1 are not appropriately compartmentalized in peroxisomes; instead, they are diffusely distributed throughout the cytoplasm, consistent with the inability of these mutant cells to import peroxisomal matrix proteins (Figure 2). The gene discussed is CAT; the disease is Zellweger syndrome.