Expression of the fragile site Xq27 in fibroblasts. II. Evidence for negative and positive clones from heterozygous females and possible relationship between frequency and phenotype.

An investigation of fibroblast clones from two females heterozygous for the Martin-Bell syndrome showed that the fibroblasts of these individuals consist of two sub-populations, one with cells expressing the fra(X)(q27) and another with cells in which this fragile site cannot be induced. This is to be expected and is in contrast to the fibroblast clones of hemizygous patients investigated earlier. The expression of fra(X)(q27) in females evidently depends on the presence of an X-linked defect with the normal allele inactivated on the other X chromosome. There may be a relationship between the mental status and the ratio of somatic cells with this defect on the active X and those cells with this defect on the inactive X chromosome in the heterozygotes. The divergence between the inactivation of the defective gene and expression of fra(X)(q27) on the late replicating X chromosome is discussed.