a segment of 129-derived chromosome (about 16 centiMorgans) remains surrounding the mutated gene (Festing, 1992) and these neighboring genes can potentially influence the phenotype of the null mice. To distinguish the role of null genes and "hitchhiking" genes, it is necessary to determine the overlap between the known QTLs for alcohol consumption and chromosomal regions of derived from the parental strains. The behavioral QTLs for alcohol preference were compiled by Saba et al. (2006) and are on Chr2 (9–49 cM and 107 cM), Chr3 (48–83 cM), Chr4 (59–75 cM), Chr5 (44 cM) and Chr9 (9–61 cM). For genes tested in our study, Cd14 and Ctsf are not on any of these chromosomes (Table 1). Il6 is located on Chr5 but is far away from the suggested QTL. B2m is located on Chr2 and Ctss is located on Chr3, but out of suggested QTL regions for these chromosomes. One gene – IL1rn - is located within a suggested QTL region on Chr2. Thus, the strong reduction of drinking seen in IL1rn mutants may be result of a combination of gene deletion and "hitchhiking" genes derived from the 129 strain.
