THE INCIDENCE OF CYSTIC-FIBROSIS IN SCOTLAND CALCULATED FROM HETEROZYGOTE FREQUENCIES

The incidence of cystic fibrosis (CF) has previously been calculated f rom epidemiological surveys and from neonatal screening. With the clon ing of the CF gene it has become possible to derive incidence figures from heterozygote frequencies, provided that the distribution of mutan t alleles among healthy carriers is the same as among affected people. We have estimated the allele frequencies for four CF mutations, Delta F508, G551D, G542X and R117H, in 14 360 unselected women undergoing a ntenatal heterozygote screening. The proportion of R117H, an allele of known mild effect, was much greater for heterozygotes than for homozy gotes. The incidence of CF was therefore calculated from the heterozyg ote frequencies of Delta F508, G551D and G542X in a larger cohort of 2 7161 successively screened women. The point estimate for the incidence of CF in the Scottish population was 1 in 1984, with 95% confidence i ntervals of 1 in 1692 to 1 in 2336.