['FORMULATION OF QUESTION: In 1991 four independent groups of geneticists succeeded in replacing the comparatively unreliable cytogenetic and indirect molecular diagnostics of the X-linked Martin-Bell-syndrome (MBS) by a more accurate approach.', 'Fine structure analysis of the disease locus at Xq 27.3 led to the discovery of the fragile X mental retardation gene I (FMR I-gene), the mutative changes of which are responsible for the disease and can easily be detected in Southern hybridizations with gene probes out of this area. The observed changes are strong amplification of a repetitive CGG-motive in exon I of the gene in patients, a more moderate one in symptom-free carriers of the disease and changes in the promotor region of the gene to a varying extent in patients and carriers.'] Does [ Discovery of the gene make the direct molecular genetic diagnosis of Martin-Bell syndrome possible ]?