Neoplastic transformation is a rare but serious complication of Paget's disease of bone (PDB), occurring in fewer than 1% of individuals with polyostotic disease. Their prognosis is poor, with less than 50% surviving 5 years. In 2016, the genetic alteration of giant cell tumor (GCT) complicating PDB was identified as a founder germline mutation (P937R) in the ZNF687 gene. However, the study population was exclusively of Italian descent, and patients of different ethnic origins were not studied. To fill this gap, herein we performed mutation analysis of ZNF687 in a GCT in the pelvis of a 45-year-old black American woman with polyostotic PDB. The P937R mutation in ZNF687 was found in her tumor but, as expected, the ancestral haplotype that characterizes the Italian GCT/PDB patients was not found. Furthermore, we identified two additional Italian GCT/PDB patients with this ZNF687 mutation, now constituting a cohort of 18 GCT/PDB cases, all harboring the identical mutation. We also searched for ZNF687 mutations in a unique collection of tumor tissues derived from Italian PDB patients, including 28 osteosarcomas (OS/PDB), 8 undifferentiated sarcomas (SRC/PDB), 1 fibrosarcoma (FS/PDB), and 1 chondrosarcoma (CS/PDB). We identified the P937R mutation in one SRC/PDB and a different ZNF687 mutation (R331W) in 1 of 28 pagetic osteosarcomas. Thus, whereas GCT/PDB pathogenesis globally seems to involve the P937R mutation in ZNF687, other neoplasms associated with PDB seem to be less related to mutations in this gene. Finally, we identified the G34W mutation in the H3F3A gene in the maxillary tumor masses of two PDB patients, defining them as conventional GCT rather than GCT/PDB. Thus, combined molecular analysis of H3F3A and ZNF687 is essential to clarify the origin and diagnosis of tumors in PDB. © 2020 American Society for Bone and Mineral Research.