COL3A1 and breast cancer: We mapped 19 overlapped DEGs between primary BC and BM samples into the STRING online tool to construct a PPI network (Figure 3), and top 5 hub genes were sorted out according to the node degree, including type I collagen α1 chain (COL1A1), lumican (LUM), type III collagen α1 chain (COL3A1), type V collagen α2 chain (COL5A2), and periosteal protein (POSTN).