GTC ENTERS INTO ATRYN® REGISTRATION LICENSE WITH BIO SIDUS ARGENTINA

FRAMINGHAM, MA – April 13, 2010 -- GTC Biotherapeutics, Inc. (“GTC”, OTCBB: GTCB) of Framingham, Massachusetts, and BIO SIDUS S.A. (“Bio Sidus”) of Buenos Aires, Argentina, announced that they have entered into a Registration License Agreement under which Bio Sidus will perform all necessary actions to obtain sales authorizations for GTC’s lead product, ATryn®, in Argentina, Uruguay and Paraguay. It is anticipated that the review process in Argentina will be completed within a fast-tracked period of no less than six (6) months from the filing of the dossier with the regulatory authorities. The dossier will be similar to that which was successfully submitted to the US Food and Drug Administration. All expenses incurred with respect to the filing, prosecution and obtaining of the sales authorizations will be borne by Bio Sidus.

A Product License agreement for the supply of material and marketing rights will be negotiated separately.

“This agreement demonstrates the continued worldwide interest in ATryn® as the only recombinant form of antithrombin. We are very excited to be working closely with Bio Sidus, a well-respected commercial biotechnology company, based in Latin America, with a strong international presence and an established interest in the production of therapeutic proteins using transgenic production technology” said Geoffrey Cox, Ph.D., Chairman, President & CEO of GTC Biotherapeutics.

“We are extremely pleased to enter into this landmark agreement with GTC which will lead to the satisfaction of unmet clinical demands in our region. The approval of ATryn® will also likely lead to acceptance by regulatory authorities, prescribers and the public of proteins derived from transgenic animals. We are confident that the transgenic production technology for targeted therapeutic proteins will be a key tool to obtain more affordable biomedicines. Biohormon®, Bio Sidus’ somatropin product obtained from the milk of transgenic cows, is also under review for marketing authorization in the near term, ” said Marcelo Argüelles, President of Bio Sidus S.A.

GTC Biotherapeutics develops supplies and commercializes therapeutic proteins produced through transgenic animal technology. ATryn®, GTC’s recombinant human antithrombin, has been approved for use in the United States and Europe. ATryn® is the first and only therapeutic product produced in transgenic animals to be approved anywhere in the world. Building upon the 2009 FDA approval of ATryn®, GTC is collaborating with its US commercialization partner, Lundbeck Inc., to develop a protocol for a pivotal study of patients with acquired antithrombin deficiencies in cardiac surgery. GTC aims to initiate a clinical trial in this indication in the second half of 2010.

In addition to ATryn®, GTC is developing a portfolio of recombinant human plasma proteins with known therapeutic properties. These proteins include recombinant forms of human coagulation factors VIIa and IX, which are being developed for the treatment of patients with hemophilia, and recombinant alpha-fetoprotein, which is being developed for the treatment of Myasthenia Gravis and Multiple Sclerosis. GTC plans to advance its human recombinant factor VIIa program into a Phase I clinical study in the first half of 2010 and its alpha-fetoprotein program into a Phase II study in Myasthenia Gravis in the second half of 2010 after securing a partner for that program.

GTC’s intellectual property includes a patent in the United States through 2021 for the production of any therapeutic protein in the milk of any transgenic mammal. GTC’s transgenic production platform is particularly well suited to enable cost effective development of proteins that are difficult to express in traditional recombinant production systems as well as proteins that are required in large volumes. Additional information is available on the GTC web site, http://www.gtc-bio.com.

Bio Sidus S.A. has developed a leading global business in the supply of high quality biopharmaceuticals in the territories of Asia, Africa, Eastern Europe and Latin America. Since the 1990’s, Bio Sidus has been marketing erythropoietin, the first recombinant protein fully developed and manufactured in Latin America. By 2005, Bio Sidus S.A. had launched six other recombinant proteins in the domestic and foreign markets.

Bio Sidus S.A. is currently developing a portfolio of ten biomolecules, and conducts research in transgenic animals and gene therapy. Its lead molecule is a recombinant human growth hormone obtained from the milk of transgenic cows. Bio Sidus is planning to initiate clinical trials with this molecule in the second half of 2010.

In the field of gene therapy, the company has completed, with promising results, its Phase I/II clinical trial GENESIS I VEGF GENE Therapy for Severe Ischemic Heart Disease with Myocardial Viability: Intramyocardial administration of plasmid-DNA coding for rhVEGF165 in patients with severe ischemic heart disease.

Bio Sidus’ activities rely on a robust IP platform which includes US process patents for some of its product portfolio, as well as other IP rights in the fields of transgenic animals and gene therapy.

This press release contains forward-looking statements, as defined in the Private Securities Litigation Reform Act of 1995, including without limitation statements related to GTC’s plans to enter into partnerships and advance its programs relating to recombinant factor VIIa, alpha-fetoprotein, ATryn and monoclonal antibodies for use as follow-on biologics. Such forward-looking statements are subject to a number of risks, uncertainties and other factors that could cause actual results to differ materially from future results expressed or implied by such statements. Factors that may cause such differences include, but are not limited to, the risks and uncertainties discussed in GTC's most recent Annual Report on Form 10-K and its other periodic reports filed with the Securities and Exchange Commission, including the risks and uncertainties associated with securing and depending upon partners, designing and conducting clinical trials and pre-clinical studies, developing new biological products and depending upon the actions of regulatory agencies and collaboration partners. GTC cautions investors not to place undue reliance on the forward-looking statements contained in this release. These statements speak only as of the date of this document, and GTC undertakes no obligation to update or revise the statements, except as may be required by law. The representations made by Bio Sidus in this statement regarding its own products are solely the responsibility of Bio Sidus. The representations made by GTC in this statement regarding its own products are solely the responsibility of GTC.