In spite of initial association of RPS4 haploinsufficiency and Turner Syndrome [14,29], the finding that X chromosome monosomy leads to a similar phenotype even in animal species that do not have an Rps4 homologue on the Y chromosome, where Rps4X might therefore be subject to inactivation, has raised some uncertainties about its role in this respect [31]. The gene discussed is RPS4X; the disease is Turner syndrome.