SERPINA1 and alpha 1-antitrypsin deficiency: Looking for another model, we came across the human AATD partial deficiency (Pi MZ phenotype) and we found that it is a powerful model to study in onetime stimulation of synthesis and block of secretion, because, due to the co-existence of two codominant encoding alleles, each hepatocyte synthesizes both the normal M and the abnormal Z AAT.