A complete-linkage, hierarchical clustering tree was estimated using the generalized association plot (GAP) (http://gap.stat. sinica.edu.tw/). GAP is implemented here as an unsupervised clustering algorithm to visually categorize all subjects based on the dissimilarity matrix, which is derived from calculating the pairwise dissimilarity of heart rate dynamics among subjects using the IBS method. Cluster-specific association was analyzed by the chisquare test to assess the frequency of b-AR polymorphisms and by the GLM model to test the differences in standard HRV indices among resulting clusters. To validate cluster-specific associations, we tested the differences in HRV and b-AR genotypes in estimated clusters based on first-and second-half ECG data.