ACP5 and Paget disease: When an MVNP transgene was targeted to the osteoclast using the tartrate-resistant acid phosphatase (TRAP) promoter, 43% of 16–20-month-old mice developed bone lesions characteristic of Paget’s disease, with abnormal osteoclasts and irregular, jigsaw-puzzle shaped bone particles that are hallmarks of Paget’s disease [21••, 22, 23••].