The XIST repressor model for the single active X.

<p>Our model depicts the putative dosage sensitive repressor(s) (yellow), which inactivate <i>XIST</i>, thereby protecting one X chromosome from inactivation in diploid 46,XX, 46,XY and 47,XXX cells, thus -directly choosing the active X (green). The non- blocked X chromosomes are inactivated by <i>XIST</i> transcription, becoming Barr bodies (red). In triploid cells (69,XXX, 69,XXY), more than one X is active because of the extra amount of the putative <i>XIST</i> repressor, contributed by the extra set of autosomes. The Y chromosome is depicted (black).