Plays a major role in tight junction-specific obliteration of the intercellular space. Directly interacts with TJP1/ZO-1, TJP2/ZO-2 and TJP3/ZO-3 (By similarity). The phosphorylated form interacts with EPCAM (PubMed:16054130). Does not interact with CD81 (PubMed:20375010). Co-localizes with EPCAM at the basolateral cell membrane and tight junction. Expressed in kidney, lung and prostate. Isoform 1 seems to be predominant, except in some normal prostate samples, where isoform 2 is the major form. Down-regulated in breast cancers, including ductal carcinoma in situ (DCIS), lobular carcinoma in situ (LCIS) and invasive ductal carcinoma (IDC) (at protein level), as well as in several cancer cell lines. Loss of expression correlates with histological grade, occurring predominantly in high-grade lesions. By androgens. Phosphorylated. Belongs to the claudin family.