Abstract The search for the basic defect in cystic fibrosis (CF) has reached a decisive stage since the recent identification of the responsible gene. Electrophysiological and biochemical research had defined the CF defect as a dysregulation of epithelial chloride channels. The putative protein product of the now identified gene shares properties with other known transport proteins, but it is not necessarily itself a chloride channel protein. Elucidation of the primary cellular defect will certainly have important aetiological and hopefully therapeutic implications. The identification of the major gene mutation already has significant consequences for genetic counselling and prenatal diagnosis. Heterozygote detection at the population le...