  In mammals, dosage compensation of X linked genes in female cells is achieved
by inactivation of one of their two X chromosomes which is randomly chosen. The
earliest steps in X-inactivation (XCI), namely the mechanism whereby cells
count their X chromosomes and choose between two equivalent X, remain
mysterious. Starting from the recent discovery of X chromosome colocalization
at the onset of X-inactivation, we propose a Statistical Mechanics model of
XCI, which is investigated by computer simulations and checked against
experimental data. Our model describes how a `blocking factor' complex is
self-assembled and why only one is formed out of many diffusible molecules,
resulting in a spontaneous symmetry breaking (SB) in the binding to two
identical chromosomes. These results are used to derive a scenario of
biological implications describing all current experimental evidences, e.g.,
the importance of colocalization.
