Figure 2A illustrates the procedure for establishing a T2DM mouse model and metformin intervention. As displayed in Fig. 2B, the concentrations of ILA, indole-3-acetaldehyde (IAAld), and KYN were significantly elevated (P < 0.05) in the serum samples of the T2DM group compared to the CON group, while the concentrations of IPA and IA were significantly reduced (P < 0.05). Here, TNFRSF9 is linked to type 2 diabetes mellitus.