As early as 1936, Penrose noted a 25% excess of males in the population of retarded individuals80. Family studies in which the males were primarily affected indicated a sex-linked defect59, which later proved to be X-linked17,55,76,84. These heterogeneous forms of X-linked mental retardation have now been classified into 3 basic syndromes: (1) Renpenning syndrome, where males are short in stature, have small heads and normal to small testes23,84; (2) Martin-Bell syndrome, where the males have a long face, prominent forehead and ears, enlarged testes (macro-orchidism) and a fragile site near the terminal region of the long arm of the X-chromosome59,87; and (3) non-specific X-linked mental retardation, where the males have neither physical abnormalities nor fragile sites on the X-chromosome39. For a more detailed description of the history and clinical aspects, see reviews42,99100,102.