Of relevant interest are the findings that, in addition to IDO induction, cytokines released in response to infection with bacterial endotoxin (lipopolysaccharide) enhance the activities of placental TDO, kynureninase and 3-HAA oxidase [66,67] and that progesterone inhibits IDO mRNA expression and enzyme activity during decidualization of the endometrium [68]. Here, IDO1 is linked to infection.