We investigated the influence of truncated O‐glycan trees, also referred to as Tn antigen, following the inactivation of C1GALT1‐specific chaperone 1 (COSMC) on the general and MMP9‐specific proteolytic processing in MDA‐MB‐231 breast cancer cells. The gene discussed is C1GALT1C1; the disease is breast carcinoma.