PTPN22 and rheumatoid arthritis: To detect potential epistasis in RA, we evaluated 1) disease associations using single SNPs (single-nucleotide polymorphisms) from 15 candidate genes and haplotypes of the PTPN22 gene, 2) gene × gene interactions among the candidate genes using the MDR method and logistic regression, and 3) gene × environmental (smoking) interactions using a case-only study design.