Nonsense mutations inactivate gene function and are the underlying cause of a large percentage of the individual cases of many genetic disorders. PTC124 is an orally bioavailable compound that promotes readthrough of premature translation termination codons, suggesting that it may have the potential to treat genetic diseases caused by nonsense mutations. Using a mouse model for cystic fibrosis (CF), we show that s.c. injection or oral administration of PTC124 to Cftr-/- mice expressing a human CFTR-G542X transgene suppressed the G542X nonsense mutation and restored a significant amount of human (h)CFTR protein and function. Translational readthrough of the premature stop codon was demonstrated in this mouse model in two ways. First, immunofluorescence staining showed that PTC124 treatment resulted in the appearance of hCFTR protein at the apical surface of intestinal glands in Cftr-/- hCFTR-G542X mice. In addition, functional assays demonstrated that PTC124 treatment restored 24-29% of the average cAMP-stimulated transepithelial chloride currents observed in wild-type mice. These results indicate that PTC124 can effectively suppress the hCFTR-G542X nonsense mutation in vivo. In light of its oral bioavailability, safety toxicology profile in animal studies, and efficacy with other nonsense alleles, PTC124 has the potential to be an important therapeutic agent for the treatment of inherited diseases caused by nonsense mutations. © 2008 by The National Academy of Sciences of the USA.