The fragile (X) chromosome demonstrable in individuals with one type of X-linked mental retardation is seldom, if ever, seen in more than 50 of cells of affected individuals. We have devised a model to explain this apparent 50 maximum, one essential feature of which is that the fragile (X) will not be seen in cells in their first division in thymidine-depleted media. The validity of our model was tested on lymphoblastoid cell lines from affected males by treating the cells with fluorodeoxyuridine (FUdR) to induce the marker and/or bromodeoxyuridine (BrdU) to determine the cell cycle. We have evidence that the fragile (X) is present in cells in the first and subsequent these observations our model is not valid and the 50 expression of the fragile site at Xq(28) and other unusual properties of this region of the X chromosome remain unexplained.
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