AHR and myalgic encephalomeyelitis/chronic fatigue syndrome: Genotyping for the Arg554Lys variant of AHR gene showed for the first time that the Lys-mutated variant was less frequent in MCS and SMCS cases, displaying similar distributions, than in FM/CFS patients and controls, and was entirely represented by the heterozygous genotype in all subgroups of cases, while the control population included both heterozygotes and homozygotes.