Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Paget disease of bone ?
    ### Response:
    Paget disease of bone is a disorder that causes bones to grow larger and weaker than normal. Affected bones may be misshapen and easily broken (fractured).  The classic form of Paget disease of bone typically appears in middle age or later. It usually occurs in one or a few bones and does not spread from one bone to another.
    