This article is available online at http://www.jlr.org human populations; these QTL often map to homologous locations in both species ( 1 ). Detecting QTL has been relatively easy. The next step of identifying causal QTL genes, however, has been more challenging, although the development of bioinformatic methods ( 2, 3 ) and improved genomic resources in the mouse ( 4 ) and genomewide association studies in humans ( 5, 6 ) are improving the success of QTL gene identifi cation.QTL that are in close proximity on the same chromosome remain particularly challenging. In such situations, it is particularly diffi cult to identify the causal QTL genes. Although congenic strains are a powerful tool to separate these QTL, they are time consuming; therefore, we used a more effi cient, genomic approach to decipher the presence of one or multiple QTL and identify the QTL genes. At least seven inbred mouse crosses ( 7-12 ) have identifi ed HDL QTL on mouse distal chromosome (Chr) 5 ( Table 1 ). However, the broad confi dence intervals and shape of the logarithm of the odds ratio (LOD) score plots suggest that at least four of these crosses (B6 × 129, NZB × SM, B6 × NZB, and B6 × CAST) may have two closely linked QTL. We used advanced intercross lines between B6 and NZB at F11, which allows the accumulation of recombination events and thus narrows QTL ( 13 ) and confi rmed that two QTL did exist between these two strains on distal Chr 5. These QTL were named Hdlq1 at 125 Mb and Hdlq8 at 113 Mb ( 11 ).In the present study, we fi rst identifi ed Scarb1 as the causal gene for Hdlq1 . We then confi rmed that Hdlq8 is an independent QTL by crossing two closely related strains that do not differ at the Scarb1 locus and thus were expected not to have a QTL at the Hdlq1 region. As expected, this cross detected Hdlq8 but had no QTL at Hdlq1. Subsequently, we used haplotype analysis, gene sequencing, expression studies, and a spontaneous mutation to demonstrate that Acads was the underlying gene for Hdlq8 .Abstract Two high-density lipoprotein cholesterol quantitative trait loci (QTL), Hdlq1 at 125 Mb and Hdlq8 at 113 Mb, were previously identifi ed on mouse distal chromosome 5. Our objective was to identify the underlying genes. We fi rst used bioinformatics to narrow the Hdlq1 locus to 56 genes. The most likely candidate, Scarb1 (scavenger receptor B1), was supported by gene expression data consistent with knockout and transgenic mouse models. Then we confi rmed Hdlq8 as an independent QTL by detecting it in an intercross between NZB and NZW (LOD = 12.7), two mouse strains that have identical genotypes for Scarb1 . Haplotyping narrowed this QTL to 9 genes; the most likely candidate was Acads (acyl-coenzymeA dehydrogenase, short chain). Sequencing showed that Acads had an amino acid polymorphism, Gly94Asp, in a conserved region; Western blotting showed that protein levels were signifi cantly different between parental strains. A previously known spontaneous deletion causes loss of ACADS activity in BALB/cBy mice. We showed that HDL le...