A group of inherited disorder characterized by mental retardation. The disorders are inherited in a X-linked manner which means that only males display the full range of symptoms whereas female carriers may have mild or no symptoms. The mental retardation may occur on its own (nonsyndromic) or be associated with other abnormalities (syndromic). The severity of the mental retardation may vary between and even within disorders. See also Mental Retardation, X-Linked