It is known that mature O‐glycosylation depends specifically on T‐synthase activity and its single chaperone‐Cosmc.31 Besides, in colonic tissues, defective core 3 O‐glycosylation could also lead to the exposure of Tn antigen.21, 32 Here we found that the expression of C3GnT that controls core 3 O‐glycosylation in cancer tissues expressing Tn antigen (Tn‐positive) was comparable to that in Tn‐negative cancerous tissues, thereby suggesting that there were only possible defects in T‐synthase and/or Cosmc (Figure 2A). The gene discussed is C1GALT1C1; the disease is cancer.