PEX2 and peroxisome biogenesis disorder: For instance, milder variant alleles of PEX2, such as PEX2E55K, in compound heterozygotes (PEX2Null/PEX2E55K) are associated with mild PBD-ZSD, with phenotypes being mild or intermediate due to residual PEX2 function (Imamura et al., 1998; Shimozawa et al., 1999).