SLC22A6 and diabetes mellitus: In order for the causal relationship of IS and endothelial injury among various uremic toxins in DKD to be elucidated, modulation of the amount of IS in an animal model of DM with renal failure will further confirm the conclusions of this study, such as decreasing IS synthesis via handling gut microbiota [71,72], reducing indole absorption by spherical carbon adsorbent AST-120 [73], increasing IS serum level by oral or intravenous extra IS supplement, or inhibiting the function of organic anion transporters 1 and 3 (OAT1/3) [74].