X inactivation in female mammals involves transcriptional silencing of an entire 
chromosome in response to a cis-acting noncoding RNA, the X inactive-specific 
transcript (Xist). Xist can also inactivate autosomal sequences, for example, in 
X;autosome translocations; but here, silencing appears to be relatively 
inefficient. This variation has been attributed to either attenuated spreading 
of Xist RNA at the onset of X inactivation or inefficient maintenance of 
autosomal silencing. Evidence to date has favored the latter. Here, we 
demonstrate attenuated spreading of Xist RNA at the onset of X inactivation in 
the T(X;4)37H X;autosome translocation. Our findings provide direct evidence 
that underlying chromosome/chromatin features can disrupt spreading of the 
primary inactivating signal.