ACP5 and bone metastasis: Furthermore, various serum markers, including tartrate-resistant acid phosphatase (TRACP) [104], procollagen type I N-terminal propeptide (P1NP) [105], and secreted Y-box binding protein 1 (sYB-1) [106], are associated with bone metastasis in breast cancer, with some markers showing potential for dynamic monitoring and improved diagnostic accuracy, though combined analysis with CTCs remains underexplored.