We hypothesize that naltrexone - but not placebo - will produce a greater clinical response during the 12 weeks of the trial in subjects with one or two copies of the Asp40 variant ("Asp40 positives") than in subjects homozygous for the Asn40 allele. Response to naltrexone will be measured by a reduction in the number of heavy drinking days (as defined by >5 drinks/day for males; >4 for females) during the 12 weeks of the trial.