Inherited Protein S (PS) deficiency is an autosomal dominant thrombotic disorder. We encountered a case of inherited type I PS deficiency following a close examination for recurrent pregnancy loss and identified the mutation responsible; a novel splice donor site mutation in intron 13 of the PROS1 gene appeared to have caused a frameshift with premature termination at amino acid +551. These results will contribute to the creation of an accurate database and define the molecular basis for PS deficiency.