HCAR1 and neoplasm: Histological analysis further demonstrated that GPR81 was strongly expressed in shNT tumors (Supplementary Fig. S3), and the number of tartrate-resistant acid phosphatase (TRAP)-positive, multinucleated osteoclasts at the tumor-bone interface was significantly decreased in shGPR81 cell-injected bone compared with that of shNT cell-injected bone (Fig. 5c–e).