Most selection programs perform bidirectional selection, with two lines selected from the same starting population for the opposite responses. Occasionally, a nonselected (quasi-randomly mated) control line is maintained. The rationale, advantages, and disadvantages of the various mating schemes have been discussed in detail elsewhere (23). Because most genetically heterogeneous mice drink very little (and therefore reach very low BECs; in S0, only 8.9% had BECs of 1 mg/mL or more) in the DID procedure, we did not deem it useful to try to breed a line selectively for low DID. Rather, we have elected to use the foundation population of HS/Npt mice as a control group for comparisons with HDID-1 mice. This colony is maintained with 48 mating pairs and, in generation 44, was genotyped using a panel of 1532 single nucleotide polymorphism markers. Allele loss was estimated at 5.1% (R. Hitzemann, personal communication). This index suggests that inbreeding by G50 is likely to be low. HS/Npt mice are currently maintained at the original animal facility (Department of Comparative Medicine, OHSU, Portland, Oregon). Both facilities are Association for Assessment and
