(A) Identification of the Gly295Val mutation in exon X of PROS1 by single strand solid-phase sequencing. A section of the autoradiograph for an affected family member (reduced free protein S antigen level) and an unaffected relative (normal free protein S antigen level) is shown. The mutation was caused by a G to T transition (arrow). Specificity for PROS1 is evident, as the second base 3′ of the mutation is a guanine only; a difference between the PROS1 gene and the pseudogene has been reported at this position.20 (B) Detection of the Gly295Val mutation by SSCP. A fragment containing exon X of PROS1 was electrophoresed in a homogenous 20% polyacrylamide gel and stained with silver nitrate. The additional band seen in affected family members heterozygous for the Gly295Val mutation is indicated with an arrow.