X-chromosome deletions in embryo-derived (EK) cell lines associated with lack of X-chromosome inactivation.

The predictions of a model for the initiation of X-chromosome inactivation based on a single inactivation centre were tested in a cytogenetic study using six different embryo-derived (EK) stem cell lines, each with a different-sized deletion of the distal part of one of the X-chromosomes. Metaphase chromosomes were prepared by the Kanda method from each cell line in the undifferentiated state and after induction of differentiation, and cytogenetic evidence sought for a dark-staining inactive X-chromosome. The results confirm the predictions of the model in that when the inactivation centre is deleted from one of the X-chromosomes neither X present in a diploid cell can be inactivated, and in addition considerably further localize the position of the inactivation centre on the X-chromosome.