Importantly, GLP-1 and GIP can cross the BBB, as can some of the GLP-1 receptor agonists that are available in the market to treat type 2 diabetes (Kastin et al., 2002; Kastin and Akerstrom, 2003; Li et al., 2020; Salameh et al., 2020; Zhang et al., 2020). The gene discussed is GLP1R; the disease is type 2 diabetes mellitus.