Killer Activation Receptors (KARs) are receptors expressed on the plasmatic membrane of Natural Killer cells (NK cells). KARs work together with inhibitory receptors (abbreviated as KIRs in the text), which inactivate them in order to regulate the NK cells functions on hosted or transformed cells. These two kinds of specific receptors have some morphological features in common, such as being transmembrane proteins. The similarities are specially found in the extracellular domains and, the differences tend to be in the intracellular domains. KARs and KIRs can have tyrosine containing activatory or inhibitory motifs in the intracellular part of the receptor molecule (they are called ITAMs and ITIMs).