INS and diabetes mellitus: Oral peroxisome proliferator-activated receptors-alpha (PPARα) agonists can be used in insulin-deficient diabetic mice (Fu et al., 2018), intraperitoneal injection of streptozotocin to induce diabetes model mice (Tomita et al., 2020a), oxygen-induced retinopathy model mice (Tomita et al., 2019), and retinal ischemia model mice (Lee et al., 2021) to promote the expression of FGF21.