Tight junctions consist on one hand of membrane proteins mediating cell to cell contacts. They include occludin, claudins, tricellulin and JAMs (junctional adhesion molecules). Scaffolding proteins on the other hand mediate signals from the surface to cytoskeletal actin filaments and activate signaling cascades of the cell [1, 3] . They include ZO-1 (Zona occludens-1), ZO-2, ZO-3, MAGI-1 (membrane-associated guanylate kinase with inverted orientation-1), cingulin and MUPP1 (multi-PDZ domain protein 1) [1, 3, 7] . Membrane proteins are divided in two groups, those with one transmembrane domain (JAMs) and those with four (claudins, occludin, tricellulin) [7] . The membrane proteins contain sequences in their carboxyterminal end with which they can attach to the scaffolding proteins [7] . One such is the PDZ domain with which the membrane proteins bind to ZO-1, ZO-2, ZO-3 or MUPP1 proteins [7, 8] . Scaffolding proteins, like ZO-1 inhibit cell proliferation by binding ZONAB (ZO-1 associated nucleic acid binding protein) thus preventing its movement to the nucleus [7] . Apg-2 (Albino and pale green 2), a protein involved in heat shock reaction, may replace ZONAB from its association with ZO-1 thus promoting its movement to the nucleus resulting in increased proliferation [7, 9] (Figure 2 ). Also ZO-2 influences cell proliferation by binding to transcription factors AP-1 (Activator protein 1) and SAF-B (Scaffolding attachment factor B) [7] . on their sequence similarity they are divided into classic and non-classic claudins [1, 12] . The former include claudins 1-10, 14, 15, 17 and 19 and the latter claudins 11-13, 16, 18 and 20-24 [1] . Claudins have four transmembrane domains, between these there are two extracellular loops (EL1 and EL2) and between inner transmembrane domains there is a short 20-residue intracellular loop [1, 12] (Figure 3 ). The intracellular carboxyterminal part contains the PDZ domains by which scaffolding proteins attach to claudins [1, 12] . The larger EL1 loop influences paracellular charge selectivity and the smaller EL2 binds the claudin molecule to the corresponding one in the neighbouring cell [12] . Claudins can associate with the same claudin or another one on the same cell membrane or with the claudin of the neighbouring cell [1, 13] . Heterodimerization between claudins can take place only between specific claudins, for instance between claudin 1 and 4 but not between claudin 1 and 2 [13] . In general, claudins 2, 7, 10, 15 and 16 increase paracellular cation permeability by forming pores in the tight junctions whereas claudins 4, 5, 8, 11, 14 and 18 have a sealing function [1] . Claudins 1, 3, 4, 5, 7, 8 and 18 are expressed in human bronchi and bronchioles but there are discrepant reports of the expression of claudin 2 [14] [15] [16] [17] [18] . In immunohistochemical studies on formalin fixed human lung tissue bronchial epithelial cells expressed claudins 2 [17, 18] . Similarly, claudin 2 has been detected in mouse lung [15] . However, in mRNA analysis of bronchial tissues it was not found [14] . Claudin 2 has been detected in skin Figure 1 In an epithelial cell layer tight junctions (marked by green) regulate the permeability of solutes and ions through the paracellular space (a violet arrow pointing downwards). Thus they function as barriers of the paracellular space and also prevent pathogens from reaching the subepithelial tissues. Tight junction also takes part in determining the polarity of epithelial cells (red arrow). Since they are located in the apicolateral part of the cell membrane they separate the apical part of the cell membrane (red) from the lateral part (yellow) preventing membrane proteins from these parts of mixing up with each other (the fence function). Interaction of claudins with scaffolding proteins ZO-1, ZO-2, ZO-3, MUPP1, cingulin and ZONAB in tight junctional structures. The arrows show interactions between specific molecules. If ZONAB is released from the association with ZO-1 it moves to the nucleus and binds to cyclin dependent kinase 4 (CDK 4) resulting in cellular proliferation. Apg-2 competes with ZONAB for the same binding site in ZO-1 thus affecting the binding of ZONAB to ZO-1 and releasing it to the nucleus. This is one example how tight junction proteins may influence the functions of the cell squamous cells and in enterocytes where it takes part in absorption of D-vitamin from the intestine [19, 20] . High amounts of claudin 2 mRNA are found in kidney, pancreas, stomach and liver tissues while neural and lymphatic tissues lacked expression [21] . In the study of Aung et al trace amounts of claudin 2 were found in the lung but the source of the expression was not determined [21] . Claudin expression appears to be the same in larger and smaller bronchi and there is no variation in claudin expression as has been found in different parts of the gut or kidney tubular segments [14, 22, 23] . Quantitative differences may, however be present and claudins appear to be located not only in the apical region of cells but also on the lateral membrane [14] . Claudin 5 is considered to be expressed mainly in endothelial cells and it takes part in the formation of the blood brain barrier [24] . It appears to be involved in the sealing function of endothelial barriers in others sites such as blood retinal or blood testis barrier [5, 6] . When claudin 5 was overexpressed in bronchial tranfectant cells it, however, made the epithelium leakier while claudins 1 and 3 made it tighter [14] . In rat alveolar cells, EGF stimulation increased the transmembrane resistance with an increased claudin 4 or 7 protein expression while claudins 3 and 5 were associated with a leakier phenotype [25] . Paradoxically then, claudin 5 appears to induce loosening of the barrier both in bronchial and alveolar cells [14, 25] .