SPP1 and clear cell renal carcinoma: Interestingly, higher OPN expression in samples from primary tumors with bone metastasis could also be related to poor prognosis, as shown in a study by Zduniak et al. [6], who investigated the expression of OPN in tissue samples from patients with clear cell renal cell carcinoma (ccRCC) and its potential role as a prognostic marker for the disease, and they provided important insights into the role of OPN in ccRCC, suggesting that OPN expression may be a useful prognostic marker for the disease.