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Factor IX
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<p>Factor IX (<a href="/facts/Enzyme_Commission_number/Y5GNLRKX">EC</a> <a href="https://enzyme.expasy.org/EC/3.4.21.22">3.4.21.22</a>), also known as Christmas factor, is one of the <a href="/facts/Serine_protease/kGfEE6hE">serine proteases</a> involved in <a href="/facts/Coagulation/vnfEevM0">coagulation</a>; it belongs to peptidase family S1. Deficiency of this protein causes <a href="/facts/Haemophilia_B/e5Afggoe">haemophilia B</a>. </p><p>It was discovered in 1952 after a young boy named <a href="/facts/Stephen_Christmas/KoyD7HzX">Stephen Christmas</a> was found to be lacking this exact factor, leading to <a href="/facts/Haemophilia/DzpVBLna">haemophilia</a>. Coagulation factor IX is on the <a href="/facts/WHO_Model_List_of_Essential_Medicines/6kmu81Xm">World Health Organization's List of Essential Medicines</a>. </p>