BINDING
SITES FOR
CTCF
(CCCTC-
binding factor)
are enriched in
escapees as well as at
the boundaries between
silenced and active regions.
The mechanism by which this
transcription factor and manager
of chromosome looping supports
escape is unclear.BINDING SITES FOR YY1
are common at escapee
promoters. YY1 is a
transcription factor that
may work in part by
infl uencing histone
acetylation, which
infl uences gene
expression.PLOTTING AN ESCAPE
The silencing of the one X chromosome in XX cells is mediated by XIST, a long noncoding RNA that is randomly
transcribed from only one X early in development. It coats the DNA and shuts down gene expression on that X. For genes
to escape, researchers hypothesize, certain sequences on the X, so-called escape elements, attract proteins that help
nearby genes evade silencing. In addition, sequences known as boundary elements and their associated proteins seem to
act as divisions between active and quiet regions. The identities of these regulators haven’t been conclusively pinned down,
but there are several suspects.
XIST RNAXIST
geneEscaped
gene’s
transcriptsYY1
CTCFEscaped
geneXist RNAHistonesEscapee
TA DSilenced
TA DGene
promoterTOPOLOGICALLY ASSOCIATED DOMAINS (TADS)distinguish
segments of the genome and typically contain genes that physically
interact. Researchers haven’t yet identifi ed the nucleic acid
sequences and proteins that set TA D boundaries, but the domains
may help delineate silenced and escaping regions on the X.© N.R.FULLER, SAYO-ART, LLC