A mouse model of pulmonary fibrosis has revealed that elevated collagen I accumulation in the lungs induces the proliferation of quiescent mouse breast cancer cells (D2.0R) that are injected into the tail vein through integrin-β1 signaling paths and the subsequent activation of the proto-oncogene tyrosine protein kinase Src, FAK, ERK, and MLCK [283]. This evidence concerns the gene PTK2 and breast cancer.