NAT2 and CYP2C9 enzymes catabolise sulfa-pyridine to excretable hydroxylamine whereas, 5 aminosalicylic acid is responsible for the pharmacotherapeutic effects in RA.4 Mutations in NAT2 and CYP2C9 lead to the accumulation of hydroxylamine which causes toxicity. This evidence concerns the gene NAT2 and rheumatoid arthritis.