The absence of vasculitis on biopsy, the presence of microvascular thrombosis, and the presence of immunofluorescent staining for IgM involving the dermal blood vessels in the absence of rheumatoid factor, cryoglobulin, and a paraprotein suggested the presence of a noncriteria antiphospholipid antibody as a cofactor in the thrombotic process. The finding of IgM antiphosphatidylserine/prothrombin complex antibodies brought clarity to the nature of the process. IgG antibodies to phosphatidylserine/prothrombin complex and IgG and IgM antibodies to phosphatidylserine were negative. The gene discussed is F2; the disease is deep vein thrombosis.