MUC21 and breast cancer: As shown in Figure 1A, 11 of 20 MUCs, including MUC1, MUC4, MUC13, MUC16, MUC21, MUC2, MUC5AC, MUC5B, MUC6, MUC9 and MUC19, were significantly upregulated in breast cancer tissues when compared with normal breast cancer tissues, and 5 of 20 MUCs, involving MUC3A, MUC15, MUC7, MUC14 and MUC18, were markedly downregulated in breast cancer samples.