CD9 and breast cancer: Regarding pathological models, CD9-knockdown suppresses the invasive and metastatic capacity of breast cancer cells in mouse xenografts (45), and CD9 deficiency reduces the severity of osteoarthritis in aging and antigen-induced arthritis models (46); however, CD9 deficiency also induces a phenotype similar to human chronic obstructive pulmonary disease (COPD) (47).