A hitherto unknown splice site defect in the protein S gene (PROS1): the mutation results in allelic exclusion and causes type I and type III protein S deficiency

A hitherto unknown splice site mutation, in the splice acceptor of intron B (tctag to tctgg), was identified in a symptomatic patient with type III protein S deficiency. The mutation co‐segregated with type I/III protein S deficiency in the patient's family. RNA analysis showed allelic exclusion of the mutant transcript in affected individuals. The apparent type III deficiency in the propositus was not associated with the protein S Heerlen variant.