In vitro reversal of fragile‐X expression by exogenous thymidine

Abstract
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.