The tissue-nonspecific alkaline phosphatase (TNSALP) gene from five German family members with childhood-type hypophosphatasia (HOPS) was analyzed using the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP)-direct sequencing method. Four novel missense mutations (T51M, R54S, L258P, and R374H) and two that had been described previously (A160T and R206W) were detected in the respective patients. Mutation A160T was detected in 3 distinct patients, and a polymorphism V505A that had been described previously was detected in the same allele as L258P mutation in 1 patient and in 2 fathers whose V505A alleles were not transmitted to the probands. No other mutations were found in 2 patients. Transient expression of the mutant proteins in COS-1 cells showed that the four novel mutations and R206W were severe alleles, whereas A160T was a moderate allele. Analysis of its enzymatic activity and genetic transmission patterns confirmed that V505A was a polymorphism. Immunoprecipitation of the transiently expressed proteins showed that levels of the 80-kDa mature form of the enzyme were diminished or absent with the severe alleles; instead, levels of high-molecular mass disulfide-linked aggregates were increased. These results suggest that in compound heterozygotes, the combination of severe and moderate alleles may combine to cause the mild phenotype seen in childhood-type HOPS.