When LEF is used for the treatment of rheumatoid arthritis (RA), individuals carrying the CYP1A2*1F CC gene are more likely to have a 9.7-fold higher risk of overall toxicity than those carrying the CYP1A2*1F A allele (Bohanec Grabar et al., 2008). Here, CYP1A2 is linked to rheumatoid arthritis.