MECP2 and Down syndrome: Studying transgenic models with known RTT-causing missense mutations on a Mecp2 null background would allow us to observe the effect of the mutation when it is the only allele present, as in RTT, whereas studying the effects of the transgene on a wild type (WT) background would enable us to study the importance of each domain to the neurotoxicity caused by MECP2 duplication syndrome.