Knock-in mice expressing various variants of factor IX are also frequently used, in particular, mice carrying human F9 with a missense mutation found in patients with severe hemophilia B (R333Q-hF9) under the mouse F9 promoter, mice with wild-type human F9-coding sequence, and mice with mutations in mouse F9 (K5A in the Gla domain of factor IX). Here, F9 is linked to hemophilia B.