In a newborn female, an abnormal karyotype, 46,XX/47,XX,+mar/47,XX,+9, was found associated with several malformations. The marker chromosome was present in 70% of peripheral blood lymphocytes, and its size appeared to be less than half of the smallest chromosomes. Several differential staining methods provided no indication as to its origin.Chromosomes isolated from EBV-immortalized lymphocytes of the patient were fractionated on a FACS-440. The marker was resolved as a sharp peak in the region close to the chromosomal debris: its DNA content seemed to be close to 40% of chromosomes 21 and 22.About 580000 minichromosomes were sorted. In order to optimize cloning conditions, a pilot cloning experiment was performed on a pool of sorted chromosomes 9, 10, 11 and 12.
A number of problems concerning both clinical and genetic or cytogenetic aspects of the fragile-X syndrome remain unsolved. In the present work, a large 5-generation fragile-X family has been clinically and cytoge-netically investigated. The results of our study indicated that an unusually high proportion of affected males was present in the family examined; all fragile-X-positive males were profoundly retarded and showed the phenotype typically described for this syndrome; moreover, we observed a variability of penetrance within the pedigree and all fragile-X-positive females in the 4th and 5th generation had some degree of mental impairment. A multistep mutation model has been proposed in order to explain some of these findings.
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