NCR1 and neoplasm: Other factors present in the tumor microenvironment have the potential to suppress NK cells, including prostaglandin E2 by inducing myeloid-derived suppressor cells (MDSCs) [19], adenosine through the binding of adenosine A2A receptors on NK cells [20], and indoleamine 2,3-dioxygenase (IDO) by catalyzing a reaction that inhibits the expression of NKp46 and NKG2 receptors [21,22].