During my early career, I identified a number of unexpected protein variants and post-translational modifications, including norleucine misincorporation, deamidation and isomerization sites, mAb heavy chain hinge cleavage and C-terminal lysine processing, O-fucosyl modifications of EGF domains, and sequence variants due to mutations in the transfected genes. I later focused on mAb structural characterization to identify sources of variation that could affect bioactivity or pharmacokinetics. These characterization studies supported our regulatory strategies by giving the Health Authorities confidence in our analytical capabilities.