In autoimmune diseases such as rheumatoid arthritis, a low dose of MTX inhibits the bifunctional purine biosynthesis protein PURH (ATIC), which blocks the conversion of aminoimidazole carboxamide ribonucleotide into inosine monophosphate, thus ceasing purine and adenosine synthesis, the latter of which is a potent proinflammatory metabolite. Here, ATIC is linked to autoimmune disease.