A smaller and somewhat less diverse group of enriched categories is associated with sensory perception. Among the most inclusive categories of this type are ''sensory perception of chemical stimulus'' (24 PSGs; P = 4.3610 239 ) and ''G-protein coupled receptor protein signaling pathway'' (39 PSGs; P = 1.4610 27 ). Previously, enrichments for such categories have been attributed primarily to olfactory receptors [4, 5] . Indeed, 15 PSGs are labeled as having ''olfactory receptor activity'' (P = 6.9610 236 ). However, eight PSGs are involved in ''sensory perception of taste,'' including five taste receptors (P = 1.4610 210 ). Interestingly, several of these are bitter taste receptors. The sense of bitter taste is critical in allowing organisms to avoid toxic and harmful substances, and extensive gene expansion of bitter taste receptors is known to have occurred during mammalian evolution [41] , possibly driven by (or helping to drive) positive selection. Bitter taste receptors under positive selection include TAS2R1, TAS2R5, and a recently expanded cluster of genes at chr12p13 (TAS2R13, TAS2R14, TAS2R42, and TAS2R49). Another PSG, TAS1R2, is a receptor of sweet and umami taste, and the PSG RTP3 is a transmembrane protein that is involved in the transport of taste receptors and apparently influences their expression. The PSGs in the ''neurological processes'' category (P = 7.5610 27 ) are dominated by olfactory and taste receptors, but they also include other types of genes. For example, TMC2 (P = 1.1610 24 ) is expressed in the inner ear and is important for balance and hearing [42] . The acid-sensing ion channel gene ACCN4 (P = 1.0610 26 ) has been implicated in synaptic transmission, pain perception, and mechanoperception [43] . SLC6A5 (P = 3.0610 24 ) is associated with hyperekplexia, a neurological disorder characterized by an excessive startle response [44] . The neuromedin receptor NMUR (P = 6.1610 24 ) is involved in the mammalian circadian oscillator system [45, 46] . Finally, the neurotensin receptor NTSR1 (P = 8.1610 24 ) mediates hypotension, hyperglycemia, hypothermia, antinociception, and regulation of intestinal motility and secretion [47] .