PTPN22 and rheumatoid arthritis: In our analysis of the Genetic Analysis Workshop 16 dataset, there was insufficient power to detect known associations with RA susceptibility at a genome-wide significance level outside of the MHC; the most significant association was p = 5.3 × 10-6 for PTPN22. Clearly, the MHC is the most influential genetic region in RA susceptibility, but extensive LD makes isolating the precise loci difficult.