We report here 7 new mutations in the ADAMTS13 gene responsible for Upshaw-Schulman syndrome (USS), a catastrophic phenotype of congenital thrombotic thrombocytopenic purpura, by analyzing 5 Japanese families. There were 3 mutations that occurred at exon-intron boundaries: 414+1G>A at intron 4, 686+1G>A at intron 6, and 1244+2T>G at intron 10 (numbered from the A of the initiation Met codon), and we confirmed that 2 of these mutations produced aberrantly spliced messenger RNAs (mRNAs). The remaining 4 mutations were missense mutations: R193W, I673F, C908Y, and R1123C. In expression experiments using HeLa cells, all mutants showed no or a marginal secretion of ADAMTS13. Taken together with the findings in our recent report we determined the responsible mutations in a total of 7 Japanese patients with USS with a uniform clinical picture of severe neonatal hyperbilirubinemia, and in their family members, based on ADAMTS13 gene analysis. Of these patients, 2 were homozygotes and 5 were compound heterozygotes. The parents of one homozygote were related (cousins), while those of the other were not. Molecular models of the metalloprotease, fifth domain of thrombospondin 1 (Tsp1-5), and Tsp1-8 domains of ADAMTS13 suggest that the missense mutations could cause structural defects in the mutants.