It is likely that endogenous or intrinsic TGase forms the G–L bonds in protein during cooking, since a variety of living organisms contain TGase in their tissues and organs1 and 4. The temperature elevation in food materials during cooking is frequently very slow, such that endogenous TGase can exert enzymatic activity for some time. Thus, it is quite reasonable to conclude that cooked or processed foods contain greater amounts of the G–L moiety than raw food materials due to the catalytic activity of endogenous TGase. That the dairy products contained no G–L bond could support such an idea, because milk itself does not contain any intrinsic TGase. Besides the intrinsic TGase, heating has been reported to induce the formation of the G–L bonds due to chemical dehydration between the γ-carboxyl group of glutamate residues and ε-amino group of lysine residues. Thus, in the presence of TGase, cooking itself results in G–L formation in proteins. In this respect, mankind has been ingesting the G–L moiety since the discovery of fire and cooking. Consequently, the safety of the G–L moiety is supported by the long-term consumption of the G–L moiety in cooked foods.