Of the 29 BXD RI lines examined, there was no apparent recombination event within the chromosome 6 Tas2r cluster. While increasing the number of BXD RI lines or the number of markers used for genotyping them would facilitate the definition of smaller QTL intervals, in this case such an effort is unlikely to permit the identification of one or a few Tas2rs involved in quinine taste. For example, we examined six lines of AXB and BXA RIs with reported recombinations around D6Mit13; a small sampling of the Tas2rs in these RI lines again indicated no recombinations within the Tas2r cluster (data not shown). Behavioral genetic approaches have been invaluable for identifying genes involved in taste function, such as the Tas1r3 gene that encodes a receptor important for sweet and umami taste [42]. Positional cloning also permitted the identification of the Tas2r responsible for the majority of variance of phenylthiocarbamide (PTC) taste sensitivity in humans [18]. In both of these cases, however, the genes linked to saccharin or PTC taste were not tightly clustered with paralogues. For bitter taste, behavioral genetic approaches may be more useful for identifying genes encoding downstream signaling molecules or components of T2R-independent transduction mechanisms. For example, a QTL for PROP avoidance has been suggested on chromosome 7 [16], and we observe a suggestive QTL for quinine taste on chromosome 8 (Figure 2); in neither case are Tas2rs found at these loci (data not shown).