CD44 is a transmembrane glycoprotein playing a key role in cel adhesion to the extracellular matrix. CD44 expression is upregulated in various cancer cells and recognized as a molecular marker for tumor-initiating cancer cells. However, the intricate correlation between CD44 and underlying biological functions is yet to be fully disclosed at molecular levels. Here, we discovererd global proteome changes induced by CD44 knockdown in the four different breast cancer cell lines by TMT based quantitative proteomics.