This application relates to a pharmaceutical composition for use in a method for treating and/or preventing a patient having ectopic ossification and/or brain tumor, wherein the patient has an active mutation in ALK2 protein which is responsible for ectopic ossification or brain tumor; an amino acid residue at position 330 of ALK2 is proline; and an active ingredient of this composition is an anti-ALK2 antibody or an antigen-binding fragment thereof comprising a property of binding to ALK2, a property of cross-linking ALK2, and a property of inhibiting BMP signal transduction.