To determine the number and frequency of mutations that occur at the cystic fibrosis locus (CF), we have examined the allele and haplotype frequencies of eight polymorphic DNA markers linked to CF in 163 Italian patients who were sub-divided according to their clinical presentations. The distribution of haplotypes for the tightly linked polymorphisms KM.19 and XV-2c differ significantly between patients with and those without pancreatic insufficiency. The haplotype found most commonly in CF chromosomes occurs much more frequently in pancreatic insufficient than in pancreatic sufficient patients. Among the 19 pancreatic sufficient patients, 6 (31.6%) show at least one copy of the rare KM.19 = 1, XV-2c = 2 haplotype, as against 16 of 138 patients (11.6%) with pancreatic insufficiency. In addition, only 5 pancreatic sufficient patients (26.3%) are homozygous for the common 2,1 haplotype, as compared with 88 patients (63.8%) with pancreatic insufficiency. These findings support the hypothesis of allelic heterogeneity at a single locus in CF and suggest that different mutations underlie the presence or absence of pancreatic insufficiency in this disorders.
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