Factor IX is a vitamin K-dependent plasma protein that participates in the intrinsic pathway of blood coagulation by converting factor X to its active form in the presence of Ca(2+) ions, phospholipids, and factor VIIIa (PubMed:8295821, PubMed:2592373, PubMed:20121197, PubMed:20121198, PubMed:1730085, PubMed:19846852, PubMed:39880037). {ECO:0000269|PubMed:1730085, ECO:0000269|PubMed:19846852, ECO:0000269|PubMed:20121197, ECO:0000269|PubMed:20121198, ECO:0000269|PubMed:2592373, ECO:0000269|PubMed:39880037, ECO:0000269|PubMed:8295821}. This is the function of Ensembl gene identifier ENSG00000101981 (F9, coagulation factor IX).