Zellweger syndrome and its milder variants--neonatal adrenoleukodystrophy and infantile Refsum disease--comprise a clinical continuum of diseases referred to as the Zellweger spectrum. Mutations in the PEX1 gene, which consists of 24 exons and encodes a AAA ATPase protein required for peroxisomal protein import, account for approximately two-thirds of the known Zellweger spectrum patient mutations. In this paper, we report on four novel PEX1 mutations and two polymorphisms in an Australasian cohort. Two of the mutations--c.1108_1109insA and c.2391_2392delTC--that lead to the introduction of a premature termination codon in exons 5 and 14, respectively, are associated with the severe Zellweger phenotype. One patient with a milder disease phenotype was a compound heterozygote for two missense mutations (I989T and R998Q), both affecting amino acids in the second, C-terminal AAA domain of the protein. PTS1 protein import levels in cultured skin fibroblasts from this patient were almost 20% of normal control levels. We have also characterized two co-segregating polymorphisms in the 5' UTR of the PEX1 gene. Based on reporter assays, the c.-137T>C polymorphism leads to reduced PEX1 expression, whereas the c.-53C>G polymorphism leads to increased expression. When present together, these regulatory polymorphisms lead to near-normal PEX1 expression. Altered PEX1 expression due to the presence of either the c.-137T>C or the c.-53C>G variant could impact on residual PEX1 function if another co-allelic mutation was present which did not completely abolish PEX1 function. It also follows that the presence of polymorphisms in the PEX1 promoter region could have implications for patients with mutations in other PEX proteins known to interact with PEX1, such as PEX6. Thus, although not deleterious in control individuals, these polymorphisms could contribute to phenotypic heterogeneity among Zellweger spectrum patients.