IDO1 and melanoma: It was also found that melanoma mediated inhibition of expression of NKp30, NKp44, and NKG2D, with subsequent impairment of NK cell–mediated cytolytic activity against melanoma cell lines was due to the functions of Indoleamine 2, 3-dioxygenase (IDO) and prostaglandin E2 (PGE2) [21].