Monoclonal antibodies are usually expressed in mammalian cell lines and are produced in several variants known as isoforms (1,–2). Microheterogeneity can result from posttranslational and enzymatic modifications as well as those caused by processing, alteration, storage, and incorrect translation of the target protein (1,3). Common sources of heterogeneity include Fc glycosylation, partial carboxypeptidase processing of heavy-chain (HC) C-terminal lysine residues (4), deamidation or isomerization (5), Fc methionine oxidation, hinge-region fragmentation (6), aggregation, and sequence variants. Sequence variants are…