ABCG2 and rheumatoid arthritis: Despite the description of frequent SNPs in other transport proteins involved in the efflux of MTX from the cell, such as ABCC1-4 and ABCG2 [181], they have not been thoroughly addressed regarding MTX effectiveness in RA; there are some reports of better response to MTX in psoriasis patients carrying SNPs for ABCC1 and ABCG2 [182] but recently three SNPs in ABCC2 and ABCG2 genes were not related to MTX response [152] and further studies are needed to clarify their true influence.