As shown for the CF mouse nasal PD [34], [35], [37], [38], the transrectal PD allows a clear-cut in vivo discrimination between CF and wild-type mice, with decreased chloride transport with near-null cAMP-stimulated response reflecting loss of function of CFTR and increased sodium transport reflecting overfunctional ENaC. The gene discussed is CFTR; the disease is cystic fibrosis.