Tissue factor, also called platelet tissue factor, factor III, or CD142, is a protein encoded by the F3 gene, present in subendothelial tissue and leukocytes. Its role in the clotting process is the initiation of thrombin formation from the zymogen prothrombin. Thromboplastin defines the cascade that leads to the activation of factor X—the tissue factor pathway. In doing so, it has replaced the previously named extrinsic pathway in order to eliminate ambiguity.