There are three main issues that limit the conclusion that can be drawn from the present study, though an effort was made to overcome all of these limitations. First, there were relatively few informative families in both our AA and EA family samples. This is due to missing genotype data for parents in some families and the low allele frequency of several POMC markers, and this reflects the lower power of ASP-based samples to detect SNP effects when analyzed by FBAT, compared to case-control samples. To overcome this difficulty and to verify the results from family-based studies, we replicated the study with both AA and EA case-control samples. Our case-control studies partially supported the results from our family-based studies, and furthermore, provided new evidence of allelic association between POMC and substance dependence. Second, in our case-control samples, there were more male cases and younger controls. To address this issue, we re-analyzed the data using backward stepwise logistic regression in which confounding factors (age and sex) were considered. The results obtained supported the findings from marker or haplotype association analyses. Third,
