GPC4 and neoplasm: After removing the irrelevant genes by PPI network analysis, a cluster composed of 11 closely connected BM genes was obtained, including SPARCL1, GPC3, LAMA1, SDC4, GPC4, ADAMTS8, LAMA2, LAMC3, SMOC1, LUM and THBS2. Most of the genes were expressed at low levels and may contribute to the reduction or degradation of BM stiffness to promote tumor invasion; similar results were reported previously (36).