Some problems in the genetics of X-linked mental retardation

S-linked mental retardation has recently become one of the most interesting genetic anomalies. Studying this group of conditions has led to many insig hts into the mechanisms involved in normal and abnormal gene actions in hum ans. Since the early 19805, the number of disease entities for which the re sponsible genes could be localized on the X chromosome has increased from y ear to year; at the Ninth International Workshop on Fragile-X-Syndrome and X-linked Mental Retardation, 199 such disease units were counted (Hamel. 19 99). Conventionally, these units were subdivided into two groups: syndromal and non-syndromal types. The syndromal types are characterized by external features, neurological signs, and/or metabolic anomalies. The non-syndroma l types do not show such specific features; here, the X-linked mode of inhe ritance is the only indicator. Due to the reduced reproduction of mentally severely retarded males? a relatively high fraction of new mutants among ca ses of a specific type must be expected. It cannot be the purpose of the pr esent short article to review sufficiently well the entire field; this woul d require a complete book. Rather, it is our intention to point to some ope n problems and possible ways for their solution. copyright (C) 2001 S. Karg er AG, Basel.