Editor—Mental retardation is a component of a large number of syndromes, most of which qualify individually as rare genetic diseases. Altogether, mental retardation affects 2-3% of the population and is unexplained in 40% of cases. According to Knightet al,1 subtle telomeric chromosomal rearrangements are responsible for approximately 1% of unexplained mental retardation (with the proportion being highest, 7.4%, in the subclass of unexplained moderate to severe mental retardation). The identification of the genes responsible will require the precise delimitation of minimum deletion regions, which relies upon the collection of a large number of cases. Because of the low expected frequency of each telomeric deletion in mental retardation, the procedure to be applied should allow the screening of many patients at a low cost.

It is not yet clear whether all chromosome ends are associated with mental retardation syndromes with similar frequency. Distal chromosome 1p36 deletions were initially detected cytogenetically because of an associated segmental imbalance.2-5 This syndrome results from both interstitial and terminal deletions of varying sizes and different breakpoints6 and the severity of the phenotype is related to the extent of the deletion.7 Clinical examination can efficiently detect a large proportion of cases, so that the number of reports of 1p36 deletions has increased significantly in the past few years which may give the false impression that this is a relatively frequent syndrome.

We show here how highly polymorphic minisatellites located within a short region can provide an efficient prescreening of samples without the need for parental samples at the initial stage. The procedure was tested here using 1p36 minisatellites and provides an estimate of the frequency of 1p36 deletions in mentally retarded patients. A selection of five highly polymorphic minisatellite probes was used to search for 1p36.3 deletions in a …