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The alpha1-antitrypsin phenotypes of two Dutch population groups (consisting of 672 and 802 individuals) were determined by the isoelectric focusing technique, which due to its recent development, has been used for the first time in large-scale phenotyping. As in other population studies on the alpha1-antitrypsin phenotype distribution, Pi M is the most frequently occurring allele. The two investigated groups exhibit remarkable differences, both to other studied groups as well as to each other. The most interesting results are probably the high frequencies of the alleles Pi- and of the recently discovered Pi MN. Comparison with phenotype studies carried out in other populations is also presented.

A new variant of alpha1-antitrypsin has been detected with the aid of isoelectric focusing on polyacrylamide slab gels. In contrast with many other variants this new alpha1-antitrypsin allele is found in 10-15% of the phenotypes examined. The electrophoretic properties of the new alpha1-antitrypsin variant in isofocusing polyacrylamide gels differ only slightly from the most common alpha1-antitrypsin allele M. On the basis of its electroforetic behaviour we propose the term MN to indicate this new protease inhibitor variant. The isofocusing technique employed, provides an easy to handle, very reproducible method for determining the alpha1-antitrypsin phenotype and can be employed for large scale screening.

During a 3-year period, newborns in the eastern part of the Netherlands were investigated for alpha 1-antitrypsin deficiency. Electroimmunoassay was used for screening, followed by Pi typing in suspected cases. In all 95 033 newborns were screened, and a mean frequency of deficiency (phenotypes Pi Z, Pi SZ, and Pi S) of 8.00 in 10 000 was found. The distribution of deficient Pi types over the area was remarkably uneven, Pi type Z being more predominant north and Pi type S south of the Rhine. Cluster areas of alpha 1-antitrypsin deficiency, with frequencies of up to 59.6 in 10 000 live births, occurred mainly in small rural communities. In urbanized areas the frequency of deficiency was lower than the mean.

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