Brother of the Regulator of Imprinted Sites (BORIS) is a mammalian CTCF paralog 
with the same central 11Zn fingers (11ZF) that mediate specific interactions 
with varying approximately 50-bp target sites. Regulated in vivo occupancy of 
such sites may yield structurally and functionally distinct CTCF/DNA complexes 
involved in various aspects of gene regulation, including epigenetic control of 
gene imprinting and X chromosome inactivation. The latter functions are mediated 
by meCpG-sensitive 11ZF binding. Because CTCF is normally present in all somatic 
cells, whereas BORIS is active only in CTCF- and 5-methylcytosine-deficient 
adult male germ cells, switching DNA occupancy from CTCF to BORIS was suggested 
to regulate site specificity and timing of epigenetic reprogramming. In addition 
to 11ZF-binding paternal imprinting control regions, cancer-testis gene 
promoters also undergo remethylation during CTCF/BORIS switching in germ cells. 
Only promoters of cancer testis genes are normally silenced in all somatic cells 
but activated during spermatogenesis when demethylated in BORIS-positive germ 
cells and are found aberrantly derepressed in various tumors. We show here that 
BORIS is also expressed in multiple cancers and is thus itself a cancer-testis 
gene and that conditional expression of BORIS in normal fibroblasts activates 
cancer-testis genes selectively. We tested if replacement of CTCF by BORIS on 
regulatory DNA occurs in vivo on activation of a prototype cancer-testis gene, 
MAGE-A1. Transition from a hypermethylated/silenced to a 
hypomethylated/activated status induced in normal cells by 
5-aza-2'-deoxycytidine (5-azadC) was mimicked by conditional input of BORIS and 
is associated with complete switching from CTCF to BORIS occupancy at a single 
11ZF target. This site manifested a novel type of CTCF/BORIS 11ZF binding 
insensitive to CpG methylation. Whereas 5-azadC induction of BORIS takes only 
few hours, derepression of MAGE-A1 occurred 1 to 2 days later, suggesting that 
BORIS mediates cancer-testis gene activation by 5-azadC. Indeed, infection of 
normal fibroblasts with anti-BORIS short hairpin RNA retroviruses before 
treatment with 5-azadC blocked reactivation of MAGE-A1. We suggest that BORIS is 
likely tethering epigenetic machinery to a novel class of CTCF/BORIS 11ZF target 
sequences that mediate induction of cancer-testis genes.