Background We identified the gene mutations in two Chinese pedigree of type Ⅰ hereditary protein C deficiency and type Ⅰ hereditary antithrombin deficiency.Methods The plasma level of protein C activity (PC∶A),protein C antigen (PC∶Ag),protein S activity,antithrombin activity (AT∶A) and antithrombin antigen (AT∶Ag) of propositi and two family members were detected using ELISA and chromogenic assay,respectively. All exons and intron-exon boundaries of protein C gene and antithrombin gene were analyzed by direct sequencing of the corresponding amplified PCR products in DNA from the propositus. Results The plasma PC∶A and PC∶Ag of propositus 1 was 26% and 1.43 mg/dl,respectively. The PC∶Ag and PC∶A of his father were normal. The decreased PC∶A level was seen in his mother and 4 of his maternal pedigree. PS∶A and AT∶A were all normal in pedigree 1 members. A C5498T heterozygous mutation in exon 3 of protein C gene,resulting in the substitution of Arg for Trp at the 15th amino acid,was identified in propositus 1 and 8 of his relatives. The plasma AT∶A and AT∶Ag of propositus 2 was 48.6% and 10.4 mg/dl,respectively. The reduced AT∶A and AT∶Ag levels were found in his father and 5 of paternal pedigree. PC∶A,PC∶Ag and PS∶A were all in normal range. A heterozygous 13387-9G deletion in exon 6 of antithrombin gene was identified in propositus 2. This mutation introduced a frameshift and a premature stop at codon 426 and existed in 6 members of pedigree 2.Conclusion The C5498T heterozygous mutation in exon 3 of protein C gene,first reported in China,leads to type I hereditary protein C deficiency. The 13387-9G deletion,a novel mutation,can cause antithrombin deficiency and thrombosis. Background We identified the gene mutations in two Chinese pedigree of type I hereditary protein C deficiency and type I hereditary antithrombin deficiency. Methods The plasma level of protein C activity (PC: A), protein C antigen (PC: Ag), protein S activity , antithrombin activity (AT: A) and antithrombin antigen (AT: Ag) of propositi and two family members were detected using ELISA and chromogenic assay, respectively. All exons and intron-exon boundaries of protein C gene and antithrombin gene were analyzed by direct sequencing of the corresponding amplified PCR products in DNA from the propositus. Results The plasma PC: A and PC: Ag of propositus 1 was 26% and 1.43 mg / dl, respectively. The PC: Ag and PC: A of his father were normal . The decreased PC: A level was seen in his mother and 4 of his maternal pedigree. PS: A and AT: A were all normal in pedigree 1 members. A C5498T heterozygous mutation in exon 3 of protein C gene, resulting in the substitution of Arg for Trp at th The plasma AT: A and AT: Ag of propositus 2 was 48.6% and 10.4 mg / dl, respectively. The reduced AT: A and AT: Ag levels were found in his father and 5 of paternal pedigree. PC:A, PC:Ag and PS:A were all in normal range. A heterozygous 13387-9G deletion in exon 6 of antithrombin gene was identified in propositus 2. This mutation introduced a frameshift and a premature stop at codon 426 and existed in 6 members of pedigree 2. Confrc The C5498T heterozygous mutation in exon 3 of protein C gene, first reported in China, leads to type I hereditary protein C deficiency. The 13387-9G deletion, a novel mutation, can cause antithrombin deficiency and thrombosis