SRR and type 2 diabetes mellitus: Although studies have shown the mixed effects of d-Ser on the rodent islet glucose-stimulated insulin secretion (GSIS) based on the d-Ser concentrations [38,39], as well as opposing outcomes associated with serine racemase expression on the islet GSIS [15,37], we observed similar d-Ser amounts in healthy and type 2 diabetes-affected islets (Figure 1C).