How prevalent is Rett syndrome in the population?

According to Chuck CurleyExecutive Director of IRSF (International Rett Syndrome Foundation) in 2007 there were nearly 4000 known cases of Rett Syndrome in the United states. DR Helen Leonard, a disability researcher from Australia's Institute for Child Health Research has collected info on the rett girls and the populations vary from country to country . Her international database, InterRett, though has collected numerous cases of Rett Syndrome. As of September 2008, InterRett had ascertained 1691 cases of Rett syndrome. These cases came from 39 different countries, with the largest proportion coming from Australia (21% ), Spain (21%), USA (18%), France (13%), Israel (5%), China (5%) and the UK (5%). Total actual number of girls per population is unknown but estimated at 1 per 10,000 to 23,000 depending on country.