The minor allele frequencies (MAF) of the three SNPs in this study, T1880C(5.3%):C602A(2.1%):T1559C(28.9%), differed from the values reported in the NCBI SNP database T1880C(1.1%):C602A(1.1%):T1559C(32.6%) submitted by HapMap project (http://www.ncbi.nlm.nih.gov/projects/SNP/snp_viewTable.cgi?pop=1410). Because population distribution and sample size are very important determinants of MAF, it is likely that the small sample size in the HapMap project (n = 45) compared to the larger sample size (n = 495) in our study, and differences in the population distribution (individuals living in the residential community at Beijing Normal University in the HapMap project vs. Beijingers in our study) are responsible for the MAF difference between the HapMap project and our study.