MECP2 and Rett syndrome: In the brain samples from female mice (3–6 weeks old) mosaic for MeCP2 expression (lacking functional MeCP2 in ∼50% of brain cells as occurs in patients with Rett syndrome), no differences were detected in any of the metabolites examined including tryptophan, kynurenine, kynurenic acid, xanthurenic acid and quinolinic acid, all of which were completely unchanged compared with wild-type littermate control animals (Fig. 5).