Numerous genes contain regulatory elements located many tens of kilobases away 
from the promoter they control. Specific mechanisms must be required to ensure 
that such distant elements can find and interact with their proper targets but 
not with extraneous genes. This review explores the connections between 
transvection phenomena, the activation of domains of homeotic gene expression, 
position effect variegation and silencers. These various examples of 
long-distance effects suggest that, in all cases, related forms of chromatin 
packaging may be involved.