BORIS (CTCFL) is the paralog of CTCF (CCCTC-binding factor; NM_006565), a 
ubiquitously expressed DNA-binding protein with diverse roles in gene expression 
and chromatin organisation. BORIS and CTCF have virtually identical zinc finger 
domains, yet display major differences in their respective C- and N-terminal 
regions. Unlike CTCF, BORIS expression has been reported only in the testis and 
certain malignancies, leading to its classification as a "cancer-testis" 
antigen. However, the expression pattern of BORIS is both a significant and 
unresolved question in the field of DNA binding proteins. Here, we identify 
BORIS in the cytoplasm and nucleus of a wide range of normal and cancer cells. 
We compare the localization of CTCF and BORIS in the nucleus and demonstrate 
enrichment of BORIS within the nucleolus, inside the nucleolin core structure 
and adjacent to fibrillarin in the dense fibrillar component. In contrast, CTCF 
is not enriched in the nucleolus. Live imaging of cells transiently transfected 
with GFP tagged BORIS confirmed the nucleolar accumulation of BORIS. While BORIS 
transcript levels are low compared to CTCF, its protein levels are readily 
detectable. These findings show that BORIS expression is more widespread than 
previously believed, and suggest a role for BORIS in nucleolar function.