The proteinase inhibitor null (Pi-) allele is a rare cause of a1 antitrypsin (AAT) deficiency. In three families, all the subjects with AAT deficiency due to PiZpresented in early childhood with recurrent chest infections and wheezing presumably related to passive smoking. In Pi-the AAT gene is present and there is no evidence for a gene deletion. In one family a restriction fragment length polymorphism (RFLP) detected with the enzyme XbaI segregates with the Pi-allele. In a family where a

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... nguineous marriage occurred, the XbaI polymorphism segregates with the normal Ml allele rather than Pi-, suggesting that Pimay have originated from Ml. In contrast, a third family and 20 normal unrelated subjects do not show the RFLP.