CTCF is a ubiquitous 11 zinc finger (ZF) protein with highly versatile 
functions: in addition to transcriptional silencing or activating in a 
context-dependent fashion, it organizes epigenetically controlled chromatin 
insulators that regulate imprinted genes in soma. Recently, we have identified a 
CTCF paralogue, termed BORIS for Brother of the Regulator of Imprinted Sites, 
that is expressed only in the testis. BORIS has the same exons encoding the 11 
ZF domain as mammalian CTCF genes, and hence interacts with similar cis 
elements, but encodes amino and carboxy termini distinct from those in CTCF. 
Normally, CTCF and BORIS are expressed in a mutually exclusive pattern that 
correlates with re-setting of methylation marks during male germ cell 
differentiation. The antagonistic features of these two gene siblings are 
underscored by showing that while CTCF overexpression blocks cell proliferation, 
expression of BORIS in normally BORIS-negative cells promotes cell growth which 
can lead to transformation. The suggestion that BORIS directs epigenetic 
reprogramming at CTCF target sites impinges on the observations that human BORIS 
is not only abnormally activated in a wide range of human cancers, but also maps 
to the cancer-associated amplification region at 20q13. The sibling rivalry 
occasioned by aberrant expression of BORIS in cancer may interfere with normal 
functions of CTCF including growth suppression, and contribute to epigenetic 
dysregulation which is a common feature in human cancer.