Serum alpha-1-antitrypsin (AAT) levels in various Pi phenotypes consisting of two alleles each, i.e . MM (left part of the figure). Serum iso-electric focusing (Pi) is one of the methods to determine the type of allele deficiency present in patients (right part of the figure). Each Pi type gives a characteristic set of bands from top to bottom on a polyacramide gel and is reported as Pi phenotype. These Pi types correlate to mean general serum AAT concentrations, but are dependent on the inflammatory status of the individual, as AAT is an acute-phase reactant protein. Clinical significant liver disease only occurs with Pi ZZ, clinical significant lung disease is associated with Pi Null, Pi SZ and PiZZ types.