CTSV and breast cancer: In previous researches, Ferreira et al found the higher level of serum YB-1 associated with more severe metastatic state.[4] In addition, in another study, cathepsin V and D were found to be correlated to breast cancer metastasis and the cancer to bone metastasis meant poor prognosis.[5] Besides above, other factors, such as interleukin-17, parathyroid hormone-related protein, and transforming growth factor-β, plastin-3, c-Src, FOXC1, and some metabolites, were also considered to be the candidates to predict bone metastasis in patients with breast cancer.[4,6–10]