The effects of four splicing mutations and one nonsense mutation on cystic fibrosis transmembrane conductance regulator ( CFTR) gene expression were investigated by reverse transcription-polymerase chain reaction analysis of mRNA extracted from nasal epithelial cells harvested from patients harbouring the mutations. We studied four subjects with 621+3A-->G, two with 2751+2T-->A, one with 296+1G-->C, two with 1717-9T-->C-D565G and seven with E822X and compared the results with CFTR mRNA from normal subjects. Our results showed that mutations 621+3A-->G, 2751+2T-->A, and 296+1G-->C, which disrupt the 5' splice donor sites of introns 4, 14a, and 2, respectively, and 1717-9T-->C-D565G, which possibly disrupts the exonic splicing enhancer sequences of exon 12 (owing to the missense mutation in cis), lead to the production of aberrantly spliced mRNA in nasal epithelial cells. Three of the splicing mutations (621+3A-->G, 2751+2T-->A, and 296+1G-->C) result in severe deficiency of normal CFTR mRNA and severe phenotype in the patients. This information is especially useful for mutation 621+3A-->G, which is found in other populations as well, and was initially reported as a polymorphism. The complex allele 1717-9T-->C-D565G results in aberrant splicing of CFTR mRNA with production of transcripts lacking exon 12 (major product), with minor amounts of transcripts revealing joint exon 11 and 12 skipping. Nonsense mutation E822X results in a severe reduction in mRNA levels to about 6% of wild type. Patients with the mutation have a severe clinical phenotype, with both the pancreatic and the pulmonary function affected.