HLA-DRB1 and rheumatoid arthritis: In summary, incorporation of haplotype clustering by the procedure of Li and Jiang [1] noticeably improves the power of the association mapping approach of Schaid et al. [3] to detect association with RA at the DRB1 locus (with adjustments to reduce signal strength), but only minimally improves power at locus D. In general, we expect clustering, in the presence of allelic heterogeneity, to improve performance of the score test and to enhance our ability to identify causative variants.