Kin selection is a widespread phenomenon through which individuals gain indirect fitness by directing altruistic behaviors toward genetically related individuals. The mechanism that permits recognition between individuals, based on genetic relatedness, needs to be highly selective and refined. Despite its critical nature, the sensory mechanisms governing this phenomenon remain incomplete. Larval zebrafish prefer kin-specific vs nonkin odors. Larvae that have been isolated or exposed to non-kin cues lose their capacity to recognize kin. Specific olfactory sensory neurons (OSNs) and the highly polymorphic major histocompatibility complex (MHC) have been implicated, however the molecular mechanisms which mediate the specificity of kin odor recognition are unknown. By combining behavioral, functional, and molecular approaches, we will unravel the molecular basis of kin recognition.