CYP2C9 and myalgic encephalomeyelitis/chronic fatigue syndrome: However, both mutated alleles were more frequent in cases than in controls as the prevalence of CYP2C9*2 was around 1.5-fold higher in patients than in controls and that of CYP2C9*3 was 7-fold, 11-fold, and 21-fold higher in MCS, SMCS, and FM/CFS patients, respectively, than in controls (Table 1).