Killer-cell immunoglobulin-like receptors (KIRs), are a family of cell surface proteins found on important cells of the immune system called natural killer (NK) cells. They regulate the killing function of these cells by interacting with MHC class I molecules, which are expressed on all cell types. This interaction allows them to detect virally infected cells or tumor cells that have a characteristic low level of Class I MHC on their surface. Most KIRs are inhibitory, meaning that their recognition of MHC suppresses the cytotoxic activity of their NK cell. Only a limited number of KIRs have the ability to activate cells. KIR molecules are highly polymorphic, meaning their gene sequences differ greatly between individuals, so that different individuals possess different arrays/repertoires of KIR genes.