Pharmaceutical compositions comprising an mRNA-loaded nanoparticle, wherein the mRNA is an in vitro transcribed mRNA and has a coding sequence at least 80% identical to SEQ ID NO: 3, and wherein the mRNA encodes a human cystic fibrosis transmembrane conductance regulator (CFTR) protein comprising the amino acid sequence of SEQ ID NO:1 are provided. The present invention is particularly useful for treating cystic fibrosis.