EE epitope, a biologically active peptide, is a fragment (314 to 319 amino acids) of the middle T antigen from mouse polyomavirus and is frequently utilized as an epitope tag. Pyroglutamyl (pGlu) peptides, which can arise spontaneously at the N-terminus from either Glutamine (Q) or Glutamic acid (E), are recognized for potentially enhancing peptide stability through the hydrophobic γ-lactam ring of pGlu, particularly against gastrointestinal proteases. Such peptides, as a normal variant, are accounted for within peptide purity assessments during HPLC analysis.