Silence Therapeutics and AstraZeneca Extend siRNA Delivery Collaboration

LONDON, April 21, 2010 - Silence Therapeutics plc (AIM: SLN) ("Silence" or the "Company")
announces that the Company and AstraZeneca (LSE: AZN) have agreed to a
one-year extension of their ongoing small interfering RNA (siRNA) delivery
collaboration. Under this collaboration, which was originally entered into in
March 2008, the companies are leveraging a number of Silence's proprietary
lipid delivery and targeting technologies to develop new and enhanced
delivery approaches for RNA interference (RNAi) therapeutics. Financial
details of the collaboration and its extension have not been disclosed.

Under terms of the agreement, both companies retain the right to
commercialize the novel delivery solutions that emerge from the
collaboration. This agreement is independent of the companies' other ongoing
collaboration which is focused on developing novel RNAi therapeutics against
specific disease targets exclusive to AstraZeneca.

"AstraZeneca's desire to extend this valuable collaboration offers
important validation for the progress that our companies have already been
able to make, as well as the fundamental siRNA delivery expertise that
Silence possesses," stated Philip Haworth, Ph.D., chief executive officer of
Silence Therapeutics. "The safe and effective delivery of RNAi therapeutics
remains one of the key challenges in this emerging space and we are excited
to be working hand in hand with a partner of AstraZeneca's stature to achieve
this goal."

The challenges of systemic siRNA delivery are well documented and include
numerous biological barriers that limit RNAi therapeutics' ability to reach
targets safely and effectively. Successfully solving this delivery problem
and enabling siRNA molecules to safely and effectively reach various tissue
sites within the body is essential to realizing the vast therapeutic
potential of RNAi.

About Silence Therapeutics plc (www.silence-therapeutics.com)

Silence Therapeutics plc (AIM: SLN) is a leading global biotechnology
company dedicated to the discovery, development and delivery of targeted,
systemic RNA interference (RNAi) therapeutics for the treatment of serious
diseases. The company possesses multiple proprietary siRNA delivery
technology platforms including AtuPLEX(TM), a system that enables the
functional delivery of siRNA molecules to targeted diseased tissues and
cells, while increasing their bioavailability and intracellular uptake. A
second, complementary delivery technology known as PolyTran(TM) uses a
library of novel peptide-based biodegradable polycationic polymers for
systemic siRNA administration. Additionally, the company has a platform of
novel short interfering RNA (siRNA) molecules, AtuRNAi, which provide a
number of advantages over conventional siRNA molecules, including increased
stability against nuclease degradation. Silence's unique RNAi assets also
include structural features for a next generation of RNAi molecules and
additional proprietary siRNA sequences against more than 50 highly valued
oncology and other disease targets.

The company's strong and diverse intellectual property portfolio includes
exclusive licenses from the University of Massachusetts on three patent
families associated with the Zamore "Design Rules," which cover broad
structural features of siRNA design for more potent next generation siRNA
sequences.

Silence Therapeutics is headquartered in London, UK, with research and
development operations in Berlin and Palo Alto, CA.

Forward-Looking Statements

This press release includes forward-looking statements that are subject
to risks, uncertainties and other factors. These risks and uncertainties
could cause actual results to differ materially from those referred to in the
forward-looking statements. All forward-looking statements are based on
information currently available to Silence Therapeutics and Silence
Therapeutics assumes no obligation to update any such forward-looking
statements.

Phil Haworth of Silence Therapeutics, +1-650-855-1514, p.haworth at silence-therapeutics.com; or Richard Potts, or Jonathan Senior, both of Nomura Code Securities, 020 7776 1200; or media, Tim Brons, tbrons at vidacommunication.com, or investors, Stephanie Diaz, sdiaz at vidacommunication.com, both of Vida Communication, +1-415-675-7400, for Silence Therapeutics