ULBP1 and neoplasm: Typical inhibitory NK cell receptor ligands include MHC class I molecules, which are often down-regulated on target tumor and infected cells, and “stress” ligands such as retinoic acid early inducible-I (Rae-I) and UL16 binding protein 1 (ULBP1), which can activate NK cells and are often up-regulated on these target cells.