Enhancers have been defined operationally as cis-regulatory sequences that can 
stimulate transcription of RNA polymerase-II-transcribed genes over large 
distances and even when located downstream of the gene. Recently, it has become 
evident that enhancers can also stimulate transcription in trans if they are 
brought into close proximity to the promoter/gene. These reports provide clues 
to the mechanism of remote enhancer action. In addition, the findings, together 
with genetic studies in Drosophila, strongly suggest that enhancer action in 
trans could underlie phenomena such as 'transvection', where one chromosome 
affects gene expression in the paired homolog.