Some Milestones in the History of X-Chromosome Inactivation

Although the discovery of X-chromosome inactivation is now over thirty years old, its mechanism is still largely an enigma. The original discovery came during the period of rapid burgeoning of knowledge in human and other mammalian genetics resulting from expansion of research due to concern over the hazards of atomic radiation after the Second World War. Thus, after the passing of several milestones, the state of knowledge of X-inactivation appears, as at the time of its discovery thirty years ago, to have reached a stage set for further rapid developments. On the one hand, there is the cloning of the Xist gene, providing a candidate for the X-inactivation center; there are models for the spreading signal, and for the maintenance and stabilization of inactivation; and there are suitable materials for testing models and candidates in the form of embryo cell lines of suitable inactivation status, genes which escape inactivation, and variations among species.