Cystic fibrosis (CF) is a disease that affects the function of specialized epithelial cells within the lungs, gut, pancreas, sweat glands, and male genital ducts. Following the cloning of the CF gene, it is possible to examine tissue-specific expression of this gene. To investigate the onset of the CF disease process, expression of the CF gene in midtrimester human fetal tissues has been analyzed by in situ hybridization using antisense RNA probes. The major site of CF gene expression at this stage of development is in the pancreas. Within the pancreas, CF mRNA is seen to be largely restricted to the epithelium of intralobular and small interlobular ducts.