γ-Secretase is a multimeric aspartyl protease that cleaves the membrane-spanning region of the β-carboxyl terminal fragment (βCTF) generated from β-amyloid precursor protein. γ-Secretase defines the generated molecular species of amyloid β-protein (Aβ), a critical molecule in the pathogenesis of Alzheimer's disease (AD). Many therapeutic trials for AD have targeted γ-secretase. However, in contrast to the great efforts in drug discovery, the enzymatic features and cleavage mechanism of γ-secretase are poorly understood. Here we review our protein-chemical analyses of the cleavage products generated from βCTF by γ-secretase, which revealed that Aβ was produced by γ-secretase through successive cleavages of βCTF, mainly at three-residue intervals. Two representative product lines were identified. ε-Cleavages occur first at Leu49-Val50 and Thr48-Leu49 of βCTF (in accordance with Aβ numbering). Longer generated Aβs, Aβ49 and Aβ48, are precursors to the majority of Aβ40 and Aβ42, concomitantly releasing the tripeptides, ITL, VIV, and IAT; and VIT and TVI, respectively. A portion of Aβ42 is processed further to Aβ38, releasing a tetrapeptide, VVIA. The presence of additional multiple minor pathways may reflect labile cleavage activities derived from the conformational flexibility of γ-secretase through molecular interactions. Because these peptide byproducts are not secreted and remain within the cells, they may serve as an indicator that reflects γ-secretase activity more directly than secreted Aβ.