In vitro reversal of fragile-X expression by exogenous thymidine.

A virtually thymidine free medium was used to test the effects of exogenous thymidine on lymphocyte expression of the fragile-X. De novo pathway thymidylic acid synthesis in the cells was blocked by FUdR. Our results suggest that in vitro exogenous thymidine is directly responsible for suppressing expression of the fragile-X. More importantly, delayed addition of exogenous thymidine can negate fragile-X expression after it has first been induced by FUdR.