1. A multispecific antibody or antigen-binding fragment thereof capable of stimulating the enzymatic activity of FIXa towards FX comprising a first antigen-binding site capable of binding to FIX (SEQ ID NO:1) and/or the activated form thereof (FIXa), and a second antigen-binding site capable of binding to FX (SEQ ID NO:2) and/or the activated form thereof (FXa).
2. The antibody or antigen-binding fragment thereof according to claim 1, wherein the first antigen-binding site is capable of binding an epitope comprising amino acid residue R338 of FIX/FIXa, and
the second antigen-binding site is capable of binding in the EGF-2 domain and/or catalytic subunit of FX/FXa.
3. The antibody or antigen-binding fragment thereof according to claim 1 or 2, wherein the first antigen-binding site is capable of binding an epitope comprising amino acid residues R338 and K341 of FIX/FIXa.
4. The antibody or antigen-binding fragment thereof according to claim 3, wherein the first antigen-binding site is capable of binding an epitope comprising amino acid residues L337, R338, S339, T340, K341 and T343 of FIX/FIXa.
5. The antibody or antigen-binding fragment thereof according to claim 1 or 2 wherein the first antigen-binding site is capable of binding an epitope comprising amino acid residues D332, R333, L337 and R338 of FIX/FIXa, and the second antigen-binding site is capable of binding in the EGF-2 domain and/or catalytic subunit of FX/FXa.
6. The antibody or antigen-binding fragment thereof according to claim 5 wherein the first antigen-binding site is capable of binding an epitope comprising amino acid residues K301, D332, R333, A334, T335, R338 and N346 of FIX/FIXa.
7. The antibody or antigen-binding fragment thereof according to any of the previous claims wherein the second antigen-binding site is capable of binding an epitope comprising amino acid residues Y230, D423 and K427 of FX/FXa.

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8. The antibody or antigen-binding fragment thereof according to any of the previous claims wherein the second antigen-binding site is capable of binding an epitope comprising one of more of amino acid residues H101, E103, R113, T116, L117, A118, T127, S227, E228, F229, Y230, E266, R287, P304, E305, L333, L419, K420, D423, R424, M426, K427 and T428 of FX/FXa.
9. The antibody or antigen-binding fragment thereof according to claim 1, wherein the antibody or antigen-binding fragment thereof competes with a reference antibody for binding to FIX/FIXa wherein the reference antibody comprises

a) a heavy chain variable domain identified by SEQ ID NO: 15 and a light chain variable domain identified by SEQ ID NO: 16, or
b) a heavy chain variable domain identified by SEQ ID NO: 17 and a light chain variable domain identified by SEQ ID NO: 18, or
c) a heavy chain variable domain identified by SEQ ID NO: 19 and a light chain variable domain identified by SEQ ID NO:20.

10. The antibody or antigen-binding fragment thereof according to any of the previous claims wherein the antibody or antigen-binding fragment thereof is a bispecific antibody.
11. The antibody or antigen-binding fragment thereof according to claim 1 comprising
a. the CDR sequences of the anti-FIX/FIXa antibody heavy chain variable
domain identified by SEQ ID NO: 177 and the CDR sequences of the anti-
FIX/FIXa antibody light chain variable domain identified by SEQ ID NO: 178,
and the CDR sequences of the anti-FX/FXa heavy chain variable domain
identified by SEQ ID NO: 179 and the CDR sequences of the anti-FX/FXa
light chain variable domain identified by SEQ ID NO: 180,
b. the CDR sequences of the anti-FIX/FIXa antibody heavy chain variable
domain identified by SEQ ID NO:181 and the CDR sequences of the anti-
FIX/FIXa antibody light chain variable domain identified by SEQ ID NO: 182,
and the CDR sequences of the anti-FX/FXa heavy chain variable domain
identified by SEQ ID NO: 183 and the CDR sequences of the anti-FX/FXa
light chain variable domain identified by SEQ ID NO: 184,
c. the CDR sequences of the anti-FIX/FIXa antibody heavy chain variable
domain identified by SEQ ID NO: 187 and the CDR sequences of the anti-
FIX/FIXa antibody light chain variable domain identified by SEQ ID NO:188,

and the CDR sequences of the anti-FX/FXa heavy chain variable domain identified by SEQ ID NO: 185 and the CDR sequences of the anti-FX/FXa light chain variable domain identified by SEQ ID NO: 186.
12. The antibody or antigen-binding fragment thereof according to any of the previous claims for use in a method of treatment of a coagulopathy or blood coagulation disorder.
13. A pharmaceutical composition comprising the antibody or antigen-binding fragment thereof according to any of the previous claims for the treatment of a coagulopathy or blood coagulation disorder such as haemophilia A with or haemophilia A with inhibitors.
14. A method of treating a subject suffering from a coagulopathy blood coagulation disorder such as haemophilia A with or haemophilia A with inhibitors, comprising administering to said subject an antibody or antigen-binding fragment thereof according to any of the previous claims.