Dentin, which is the part of the tooth under the enamel, is mainly composed of collagen. Collagen makes up 90% of the dentin organic matrix. The most common type of collagen found in dentin is type 1, however, dentin also contains type 3 collagen and traces of type 5 collagen. Accumulation of advanced glycation end-products in dentin is caused by a chemical reaction where sulfhydryl protein linkages are replaced by glucose. This reaction is known as nonenzymatic glycation. Specifically, the buildup of advanced glycation end-products causes crosslinking between amino acids polypeptides in the collagen. This structural change in the collagen modifies the mechanical properties of dentinal collagen. In 2016, a study was conducted to see if the accumulation of advanced glycation end-products contributed to the progression of cavities. This study discovered that ultrasound of type 1 collagen and the total amount of advanced glycation end-products varied noticeably in regions of the teeth that contained cavities. The study concluded that an increased amount of advanced glycation end-products could contribute to the progression of cavities. This was determined by noting how the fluorescence lifetime was shorter in the areas of the teeth that had cavities than in areas that did not contain cavities.