Approximately one-third of the alleles causing genetic diseases carry premature termination codons (PTCs). Therapeutic approaches for mutations generating in-frame PTCs are aimed at promoting translational readthrough of the PTC, to enable the synthesis and expression of full-length functional proteins. Interestingly, readthrough studies in tissue culture cells, mouse models, and clinical trials revealed a wide variability in the response to the readthrough treatments. The molecular basis for this variability includes the identity of the PTC and its sequence context, the chemical composition of the readthrough drug, and, as we showed recently, the level of PTC-bearing transcripts. One post-transcriptional mechanism that specifically regulates the level of PTC-bearing transcripts is nonsense-mediated mRNA decay (NMD). We have previously shown a role for NMD in regulating the response of CF patients carrying CFTR PTCs to readthrough treatment. Here we describe all the protocols for analyzing CFTR nonsense transcript levels and for investigating the role of NMD in the response to readthrough treatment. This includes inhibition of the NMD mechanism, quantification of CFTR nonsense transcripts and physiologic NMD substrates, and analysis of the CFTR function.