CLDN5 is tightly regulated at the blood-brain-barrier and is compromised in several neurological diseases (Hashimoto et al., 2023), while in breast-cancer models, overexpression of CLDN20 increases trans-epithelial electrical resistance (TEER) (Martin et al., 2013), both tissue-specific roles highlighting the complex nature of claudins. Here, CLDN5 is linked to breast cancer.