Claudin-2 is a tight junction membrane protein that can create a pathway selective for small positively charged ions between epithelial cells. Its presence can make a junction more conductive without creating an indiscriminate hole through the epithelial sheet.
Researchers expressed claudin-2 in an epithelial layer that initially had low conductance between cells. Conductivity increased, and comparison of different salts showed preferential passage of cations over anions.[ref]
Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells by Salah Amasheh and others. 2002, abstract: transfected monolayers and relative ion permeabilities
This is a useful counterexample to treating every claudin as a simple seal: changing the molecular composition of a tight junction can change both how much passes and which substances pass. New to topics? Read the docs here!