Hereditary protein S deficiency is a risk factor for developing recurrent venous thromboembolic disease and is caused by a defect in the protein S 1 (PROSl) gene. Identification of the mutation in the PROSl gene can overcome diagnostic uncertainty in family members with borderline protein S levels. We describe a novel nonisotopic method for molecular diagnosis of protein S deficiency, using fluorescein-labeled amplification and sequencing primers. As a first step, all exons of the PROS 1 gene are selectively amplified, and heteroduplex analysis is performed. As a second step, all exons are analyzed by direct sequencing. Using this method, we have characterized the molecular defect in two Belgian families with hereditary protein S deficiency type I: a frameshift mutation in exon XIV (1881insTC) and a missense mutation caused by a T-to-C transition, resulting in substitution of Leu405 by Pro (L405P). © 1997 S. Karger AG, Basel.