A total of 57 BRGs were identified as co-expressed genes with a correlation coefficient greater than 0.3 in breast cancer tissues, including genes that encode bone matrix proteins (ASPN, COMP, CSPG2/VCAN, DSPG3/EPYC, POSTN, SPARC, SPOCK, COLs), extracellular matrix (ECM)-degrading enzymes (MMPs, ADAMs, ADAMTSs, CTSK, PLAU/uPA) and their inhibitors (TIMPs), osteoblast cadherin (OB-cadherin/CDH11), osteoblast transcription factors (RUNX2), and cytokines (SCGF; Figure 1A). The gene discussed is CTSK; the disease is breast cancer.