MECP2 and atypical Rett syndrome: Because MECP2 undergoes XCI, this last feature allows the separation of cells that express the wild-type MECP2 from the active X from those cells that express the mutant MECP2 from the active X. To identify downstream targets of MeCP2, we compared global gene expression patterns in matched pairs of clonally derived mutant or wild-type MECP2-expressing lymphocyte cultures from two girls with classic Rett syndrome.