This motivated us to investigate in step 1 whether specific genetic polymorphisms of oxidative stress markers, glutathione S-transferase Mu 1 (GSTM1), glutathione S-transferase pi 1 (GSTP1), SOD2, CAT, MPO, and EPHX1 are related to a higher urine concentration of phthalates and then in step 2 whether these genes may modify effects of phthalates on asthma. The gene discussed is CAT; the disease is asthma.