The isolation of cystic fibrosis gene at the CF locus assigned to the long arm of chromosome 7 band q31 and definition of its protein product named CFTR (cystic fibrosis transmembrane conductance regulator) permits to understand the basic defect in this inherited disorder known as cystic fibrosis (CF) or mucoviscidosis. A variety of mutations of CF gene was revealed and the most common, a deletion of the 3 nucleotides that encode phenylalanine (Delta F508) with the variable incidence among the different ethnic groups of CF patients was delineated. CF is a variable disease and genetic testing can be useful to explain this variation but to date the phenotype-genotype correlation is not clarified. Polymerase chain reaction (PCR) amplification is used to test CF gene in CF patients and their families but is not sufficiently genetically informative to population screening for carrier detections. Recently identified glycoprotein encoded by CF gene is responsible for the regulation of the membrane chloride channel of epithelial cells and the experiments used retro-virus-mediated gene transfer demonstrated complementation CF defect in vitro. The way for gene therapy in this disease is open. An alternative approach to use the germ line cells to CF gene therapy is prerequisite of the development of the preimplantation preconception genetic CF diagnosis. The researchers managed already to express human CFTR gene in vivo in cotton rats lungs through the viral delivery system. It will be generalized into the airways of CF patients with hope that normal CFTR will reverse the physiological defect in CF cells.