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Null alpha1-antitrypsin (alpha1AT) alleles represent the end of a continuum of variants associated with profound alpha1AT deficiency and an increased risk of emphysema. This study characterizes the molecular basis of QOclayton, a new example of an alpha1AT null allele arising from a mutational hot spot in the alpha1AT gene. The QOclayton allele is identical to the normal M1(V213) alpha1AT allele except for an insertion of a cytosine. This insertion occurs in the alpha1AT sequence which normally has seven cytosines corresponding to amino acid residues 360 to 362. The QOclayton mutation is located in the same reiterated DNA sequence as the alpha1AT QObolton deletion mutation and the insertion mutation allele QOsaarbruecken. The QOclayton cytosine insertion causes a 3' frameshift and results in the formation of a termination codon at residue 376, the same consequence as the alpha1AT QOmattawa mutation (L353 T-insertion with a 3' frameshift). To determine the molecular mechanisms responsible for the absence of alpha1AT associated with the QOclayton gene, an in vitro model of QOclayton was established using Chinese hamster ovary cells (CHO) transfected with the QOclayton gene. These cells were evaluated for alpha1AT mRNA expression, protein synthesis and secretion. Although the QOclayton gene expresses a similar amount of alpha1AT mRNA as compared with the normal alpha1AT gene, no QOclayton protein is secreted. Protein trafficking and double-label immunofluorescence demonstrate that the QOclayton protein is retained in the rough endoplasmic reticulum or pre-Golgi compartment and is degraded (t1/2 = 6.5 h). Since QOmattawa, QObolton, and QOsaarbruecken have similar termination sites in the alpha1AT mRNA, they may share a similar intracellular fate.