The fact that BDNF levels in MeCP2 mice did not increase in the first weeks after birth as in WT mice (Kolbeck et al., 1999; Chang et al., 2006) as well as a more severe phenotype of MeCP2 null mice crossed with BDNF-deficient mice or an ameliorated phenotype if MeCP2 null mice were crossed with mice overexpressing BDNF (Chang et al., 2006) pointed towards an influence of BDNF on the course of Rett disease. Here, BDNF is linked to Rett syndrome.