The ATP-binding cassette transporter (TAP) proteins are functionally relevant candidates for predisposition to systemic lupus erythematosus (SLE) by virtue of their role in autoantigen presentation and location in the major histocompatibility complex (MHC). We tested if variation in the TAP genes (TAP1 and TAP2) is associated with SLE. We genotyped tag single nucleotide polymorphisms (SNPs) and performed family-based association analysis on 390 Caucasian pedigrees. We found significant evidence of association between TAP2 and SLE (rs241453, P=1.33 x 10(-6)). Conditional logistic regression analysis suggests that this TAP2 effect is separate from the HLA-DRB1 alleles. Our analyses show that both rs241453 (P=1.6 x 10(-4)) and HLA-DRB1*03xx (P=2.3 x 10(-4)) have significant autonomous effects not due to linkage disequilibrium. Moreover, these loci exhibit a significant statistical interaction (P<1.0 x 10(-6)), demonstrated by an increase in the odds ratio for the TAP2 association from OR=2.00 (95% confidence interval (CI)=1.17-3.42) in HLA-DRB1*03xx-negative subjects to OR=4.29 (CI=1.88-9.76) in the subjects with at least one HLA-DRB1*03xx allele group. We report the largest association study of the TAP genes with SLE to date, and the first to test for its separate effect and interaction with the HLA alleles consistently associated with SLE.