Furthermore, RA-DB patients carrying at least one abnormal CFTR allele but not fulfilling the diagnostic criteria for CF more frequently had chronic sinusitis, a trend toward more severe pulmonary involvement, and CFTR protein dysfunction, as demonstrated by the lower nasal potential difference of these patients than of patients with both RA and DB but without CF/CFTR-RD mutations [16]. This evidence concerns the gene CFTR and cystic fibrosis.