As aforementioned, accumulation of kynurenine metabolites due to IDO overexpression is well described during acute and chronic kidney injury [40,72], and a higher xanthurenic acid (and resulting lower 3-hydroxykynurenine/xanthurenic acid ratio) has been shown to associate with the lower functional vitamin B6 status and long-term mortality in kidney transplant patients [73]. This evidence concerns the gene IDO1 and kidney transplant.