PCDH19 and Williams syndrome: Other than Down syndrome, the commonest known cause with a birth prevalence of ∼1/1,000 live births (101), other much rarer syndromes include Angelman, ARX, Coffin-Lowry syndrome, Cornelia de Lange, Fragile X, PCDH19, Prader–Willi, and Rett and Williams syndromes (102).