The gene encoding AAT is located on the long arm of chromosome 14 within the q31-32.3 region and it named SERPINA1 or Protease inhibitor (Pi). There are many hereditary variants of the Pi gene. The most frequently occurring family of ‘normal’ alleles is described as PiM. The most common alleles leading to AATD are PiZ and PiS. A series of null alleles (designated PiQ0 or Pinull) is associated with the most severe deficiency, producing no active AAT or less than 1% of the normal amount in the plasma.
There are more than 100 different allelic variants of AAT but most of these are very rare.