The inactivation of the fragile X chromosome in female carriers of the Martin Bell syndrome as studied by two different methods

Female heterozygotes for the fragile X syndrome show variable levels of mental handicap from normal to severely retarded. The degree to which they are affected may depend upon whether the fragile or the normal X chromosome is preferentially inactivated, but one of the problems with the use of BUdR for the study of Lyonisation in fragile-X heterozygotes is that it reduces the level of expression of the fragile site. Results obtained by this method will be biased if the suppression occurs preferentially in either the active or the inactive X chromosome. To confirm that BUdR does not preferentially cause repair of the fragile site on either the late or the early replicating X chromosome, a comparison was made between the percentage of active or early fragile-X obtained using BUdR, and that obtained using tritiated thymidine in cells from the same heterozygote.