Genetic deletion of Pex2, Pex5, or Pex13 in mice causes profound and global peroxisome deficiency, and is associated with growth delay, severe hypotonia, and death shortly after birth, similar to Zellweger syndrome in humans (Baes et al., 1997; Faust and Hatten, 1997; Maxwell et al., 2003). Here, PEX5 is linked to Zellweger syndrome.