It is a Vitamin K-dependent factor synthesized in the liver. It functions as a cofactor to Protein C in the inactivation of Factors Va and VIIIa. Congenital deficiency of Protein S A. Inheritance is autosomal dominant.
Homozygotes often develop neonatal purpura fulminans shortly after birth.
Heterozygotes can also develop thrombosis. There are two types of congenital Protein S deficiencies 1. Type I - Decreased free Protein S, but adequate bound levels. Arterial thrombosis is not increased. B. Seen in patients taking warfarin.