The clotting factors are numbered I through XIII, yet only 12 are recognized. Which number is no longer used?

1. The clotting factors were numbered I through XIII in the order of their discovery. 2. Factor VI was once believed to be distinct but is now thought to be identical to factor V. 3. Rather than renumber everything, factor VI was left as a placeholder. 4. Thirteen numbers with one placeholder therefore leaves 12 recognised factors. 5. Factor IV is the calcium ion and is very much in use across the entire process. 6. Factor V is proaccelerin and remains in use, so retiring it rather than VI is the natural trap. 7. Factor XIII stabilises the fibrin clot and is the last active factor rather than an unused one. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_