Distribution of the six alpha 1-antitrypsin (PI) subtypes from a sample of blood donors.

The PI M phenotype of alpha 1-antitrypsin was classified into six subtypes determined by three alleles: PIMI, PIM2 and PI*M3. Phenotypes were classified by isoelectric focusing and their distribution agreed closely with Hardy-Weinberg expectations. The proportion of heterozygotes was 53%, which more than trebles the heterozygosity without subtyping, and is comparable to a recent estimate from U.S. whites.