CFTR and cystic fibrosis: In wild type mice the transepithelial import of chloride is rate limiting, but at the 10–20 fold lower concentrations in our CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu mice the Cftr mediated current is the rate limiting step for chloride ion flow.