MUC1 and breast cancer: there was a negative correlation between anti-MUC1 IgG and CA15-3 antigen in stage IV breast cancer when positive CA15-3 antigen and/or anti-MUC1 IgG were selected (r=−0.417; P=0.0044). The positive anti-MUC1 IgG with positive Ca15-3 antigen was more common in stage IV breast cancer, compared with early-stage breast cancer (χ2 = 4.629; P=0.031), however, Ca15-3 antigen positivity was higher in stage IV breast cancer, compared with early-stage breast