Peroxisomal disorders are divided into the two categories: (1) disorders of peroxisome assembly or peroxisome biogenesis disorders (PBD) and (2) single protein defects (it may be better to refer to these as single protein defects, since ABCD1 encodes a protein that appears to be a transporter and not an enzyme) (Raymond, 2001). In the first, the peroxisome fails to form and there are abnormalities of multiple peroxisomal enzymes. It is now understood that these disorders are defects in protein importation or membrane incorporation. The PBDs can be further divided by their clinical and biochemical features into the Zellweger spectrum disorders (ZSD) and rhizomelic chondrodysplasia punctata (RCDP).