CLDN4 and breast cancer: Overexpression of CLDN4 has been reported in various cancers, such as gastric cancer [6,7,8], pancreatic cancer [9,10,11], colorectal cancer [12,13], breast cancer [14,15] (especially triple-negative breast cancer [16,17]), oral squamous cell carcinoma [18], ovarian cancer [19], bladder cancer [20,21], non-small cell lung cancer [22], and cholangiocarcinoma [23].