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Salah Amasheh studies the selective barrier formed by epithelium. A 2002 study led by Amasheh tested whether claudin-2, a tight junction protein, could change the passage of ions between neighboring biological cells. The investigators introduced Complementary DNA encoding claudin-2 into a normally resistant cultured kidney epithelium, checked where the protein appeared, and compared electrical measurements with control cultures.^[ref]

Claudin-2 expression increased conductivity through the spaces between biological cells by a factor of 5.6 and favored cations over anions. Transport of several larger test substances did not increase. The result illustrates a selective channel rather than an indiscriminate hole: greater passage of one class of ions need not mean greater passage of every dissolved substance. Combining localization, controls and measurements of different solutes made the interpretation stronger than an electrical measurement alone.^[ref]