Table 1.

Distinctive characteristics of claudins

Claudin	Distinctive characteristics
1	Present in high resistance epithelia (collecting segment), absent in leaky epithelia (proximal tubule)
	Crucial for the mammalian epidermal barrier
	Mutations cause neonatal sclerosing cholangitis
	Prognostic value in colon and thyroid cancer
2	Present in leaky epithelia (proximal tubule) and absent in tight epithelia
	Present in the choroids plexus epithelium
3	Present in the tighter segments of the nephron
	Up-regulated in ovarian, breast, prostate and pancreatic tumors
4	Induces selective decrease in sodium permeability
	Present in the tighter segments of the nephron
	Alternative name: CPE-R
	Up-regulated in ovarian, breast, prostate and pancreatic tumors
5	Frequently deleted in velo cardio facial syndrome
	Constitutes TJ strands in endothelial cells and it is transiently expressed during the development of retinal pigment epithelium
6	Present in embryonic epithelia
	Its overexpression in transgenic mice generates a defective epidermal permeability barrier
7	Down regulated in head and neck squamous cell carcinomas
	Upregulated in stomach cancer
8	Present in the tighter segments of the nephron
10	Prognostic value in hepatocellular carcinoma (recurrence)
11	Present in oligodendrocytes and sertoli cells; also named OSP
14	Expressed in the sensory epithelium of the organ of Corti Mutations cause autosomal recessive deafness
15	Present in endothelial cells
16	Critical for Mg2+ and Ca2+ resorption in the human thick ascending limb of Henle
	Mutations cause familial hypomagnesaemia
18	Expressed in the lung and stomach

Claudins 9, 12, 13, 17, 19-24: Insufficient data. CPE-R: Clostridium perfringens enterotoxin receptor; OSP: Oligodendrocyte sertoli protein.