The peroxisome biogenesis disorders (PBD) are a heterogeneous group of lethal, autosomal recessive diseases that are characterized by defective import of peroxisomal matrix proteins. The clinical phenotypes include those of the Zellweger Syndrome (ZS) spectrum and classical RCDP. RCDP patients comprise a single, large complementation group (CG), CG11, A candidate gene for CG11, PEX7, was identified in yeast and encodes the receptor for peroxisomal matrix proteins with the type 2 peroxisome targeting signal(PTS2). Taking advantage of the conservation of peroxisome function throughout evolution, we used the yeast protein to probe the database of expressed sequence tags and identified both the human and murine PEX7 genes. We show that expression of murine Pex7 protein rescues the PTS2 import defect characteristic of CG11 cells. In a series of 36 RCDP probands, we found two inactivating PEX7 mutant alleles: one, L292ter, was present in 26 probands and associated with the classical, severe phenotype. The high frequency of L292ter may be due to a founder effect and may account, in part, for the relative phenotypic homogeneity of RCDP. The second mutant allele, A218V, was found in the only two patients in our series with a mild phenotype. We suggest that, similar to mutations described in the fibroblast growth factors, heterogeneity of PEX7 mutations may produce phenotypes distinct from RCDP.