This system can indicate that this mouse is John Smith. How does it also tell that he's your never-before-encountered brother? The closer the relative, the more similar their cluster of MHC genes and the more similar their olfactory signature. Olfactory neurons in a mouse contain receptors that respond most strongly to the mouse's own MHC protein. Thus, if the receptor is maximally stimulated, it means the mouse is sniffing its armpit. If near maximally stimulated, it's a close relative. Moderately, a distant relative. Not at all (though the MHC protein is being detected by other olfactory receptors), it's a hippo's