Antisense RNA (asRNA) is 
a single-stranded RNA that 
is complementary to a messenger RNA (mRNA)
strand transcribed within a cell.
Some authors have used the term micRNA (mRNA-interfering
complementary RNA) to refer to these RNAs
but it is not widely used.[1]
Antisense RNA may be introduced into a cell
to inhibit translation of a complementary
mRNA by base pairing to it and physically
obstructing the translation machinery.[2]
This effect is therefore stoichiometric.
An example of naturally occurring mRNA antisense
mechanism is the hok/sok system of the E.
coli R1 plasmid.
Antisense RNA has long been thought of as
a promising technique 
for disease therapy; the only such case to
have reached the market is the drug fomivirsen.
One commentator has characterized antisense
RNA as one of "dozens of technologies that
are gorgeous in concept, but exasperating
in [commercialization]".[3] Generally, antisense
RNA still lack effective design, biological
activity, and efficient route of administration.[4]
The effects of antisense RNA are related with
the effects of RNA interference (RNAi).
The RNAi process, found only in eukaryotes,
is initiated by double-stranded RNA fragments,
which may be created by the expression of
an anti-sense RNA followed by the base-pairing
of 
the anti-sense strand to the target transcript.[5]
Double-stranded RNA may be created by other
mechanisms (including secondary RNA structure).
The double-stranded RNA is cleaved into small
fragements by DICER, and then a single strand
of the fragment is incorporated into the RNA-induced
silencing complex (RISC) so that the RISC
may bind to and degrade the complementary
mRNA target.[6] Some genetically engineered
transgenic plants 
that express antisense RNA do activate the
RNAi pathway.[7] This processes resulted in
differing magnitudes of gene silencing induced
by the expression of antisense RNA.
Well-known examples include the Flavr Savr
tomato and two cultivars of ringspot-resistant
papaya.[8][9]
Transcription of longer cis-antisense transcripts
is a common phenomenon in the mammalian transcriptome.[10]
Although 
the function of some cases have been described,
such as the Zeb2/Sip1 antisense RNA, no general
function has been elucidated.
In the case of Zeb2/Sip1,[11] the antisense
noncoding RNA is opposite the 5' splice site
of an intron in the 5'UTR of the Zeb2 mRNA.
Expression of the antisense ncRNA prevents
splicing of an intron that contains a ribosome
entry site necessary for efficient expression
of the Zeb2 protein.
Transcription of long antisense ncRNAs is
often concordant with the associated protein-coding
gene,[12] but more detailed studies have revealed
that the relative expression patterns of the
mRNA and antisense ncRNA are complex.
