Research on a wide range of cytokines in patients with rheumatoid arthritis in relation to the phenotype of CYP3A4—quantified as the concentration of an endogenous metabolite (4β-hydroxycholesterol) in the serum of patients treated with inhibitors of TNF—has detected significant associations between cytokine levels and the CYP3A4 phenotype. The gene discussed is CYP3A4; the disease is rheumatoid arthritis.