NK1.1, also known as CD161 or KLRB1, is a type II transmembrane C-type lectin-like receptor that is primarily expressed on the surface of natural killer (NK) cells and certain subsets of T cells, including both CD4+ and CD8+ T cells. It is particularly prevalent on adult T cells with a "memory" antigenic phenotype. The expression of NK1.1 is associated with the cytotoxic function of CD16+ NK cells, and its ligation with its ligand LLT1 can inhibit NK cell cytotoxicity and cytokine secretion. This marker is commonly used in the identification and study of NK cells, especially in mouse models, where it is a key indicator for distinguishing NK cells from other immune cell populations.