Excessive fibrin deposits at the fetal-maternal interface early during development, such as in Procr (or Epcr) KO embryos results in severe placental thrombosis and lethality at E10.5 (Gu et al., 2002); whereas in Wnt2 KO embryos showed fibrin deposits between E14 and E18 with maternal blood accumulation in the labyrinth zone, resulting in 50% viability and smaller pups at birth (Monkley et al., 1996). The gene discussed is WNT2; the disease is deep vein thrombosis.