Over the last decade, Xq28 duplications including MECP2 (methyl CpG-binding protein 2; OMIM 300005), have been identified in approximately 140 male patients presenting with hypotonia, severe developmental delay/intellectual disability (DD/ID), limited or absent speech and ambulation, and recurrent respiratory infections [1-25]. The gene discussed is MECP2; the disease is dentin dysplasia.