CLDN6 and breast cancer: Under hypoxic conditions, the accumulation of hypoxia-inducible factor 1 (HIF-1) promotes CLDN6 transcription, and increased CLDN6 weakens HIF-1α protein stability by reducing SUMO-specific protease 1 expression and preventing the deSUMOylation of HIF-1α, ultimately leading to HIF-1α degradation and breast cancer metastasis suppression (139).