CLDN2 and breast cancer: However, we found that CLDN1, 5, 6, 9, 10, 11, 15, 19, and 23 had higher promoter methylation levels in BC tissues than in normal tissues, whereas CLDN2, 4, 7, 8, 12, 16, 17, 18, and 20 had lower promoter methylation levels in BC tissues than in normal tissues (Supplementary Figure 1).