MECP2 and atypical Rett syndrome: Mecp2-deficient mice (a mouse model of Rett Syndrome that phenocopies many human symptoms) showed both a deficiency of NA populations (reduced number of NA neurons in C2/A2 and C1/A1 group) and highly variable respiratory rhythm at around 4–5 weeks of age (Roux et al., 2007; Viemari et al., 2005).