**3.5 Heparin Cofactor II (HCII) deficiency**

HCII was first detected and isolated in the early 80s (Tollefsen & Blank, 1981, Tollefsen et al., 1982). It specifically inhibits thrombin with less affinity than AT and therefore it may be considered as a second line inhibitor of thrombin (Ehsan & Plumbley, 2002). A number of cases with HCII deficiency were reported to have VTE, but many cases remained asymptomatic (Ehsan & Plumbley, 2002; Laffan & Manning, 2002b). More studies are needed on larger number of cases to determine any significant effect of this defect in causing VTE.
