MECP2 and atypical Rett syndrome: In our study, among the seven neonates with hypotonia under the category of RETT syndrome, six carried mutations in the MECP2 gene, and the other exhibited a homozygous missense mutation, c.216T>A in CDKL5. Among the six neonates with hypotonia who carried mutations in the MECP2 gene, three carried the same heterozygous missense mutation, c.602C>T, and exhibited atypical phenotype of hypotonia and/or difficult feeding.