What is Pi (proteinase inhibitor) null or PiQO?: a problem highlighted by the alpha 1 antitrypsin Mheerlen mutation

alpha 1 antitrypsin deficiency is associated with predisposition to the development of pulmonary emphysema and childhood cirrhosis. There are two common deficiency alleles in the European population, proteinase inhibitor (Pi) Z and S. In addition, there are rare Pinull or QO variants which can be difficult to diagnose. A family assigned as having the PiQO allele by AAT protein quantification and isoelectric focusing was shown by DNA sequencing to have the PiMheerlen mutation (Pro369-Leu). This highlights the difficulties of diagnosis of PiQO.