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For Immediate Release

CATALYST BIOSCIENCES SELECTS DRUG DEVELOPMENT CANDIDATE IN FACTOR VIIA PROGRAM FOR HEMOPHILIA

South San Francisco, CA, May 6, 2008 - Catalyst Biosciences, Inc., a pioneer in the discovery and development of engineered proteases known as Alterase™ therapeutics, announced that it has selected CB 813, an improved, second-generation variant of human coagulation factor VIIa, as a development candidate for the treatment of acute bleeding in hemophilia patients.  CB 813 was created using Catalyst’s proprietary therapeutic protease platform technologies and know-how.

CB 813 is designed to substantially enhance clot-generating activity at the site of bleeding and therefore achieve clinical efficacy with fewer and lower doses than current therapy.  In established hemophilia models of acute bleeding, CB 813 has demonstrated a significant improvement in potency compared with the marketed recombinant factor VIIa product, NovoSeven® and a competing, second-generation product, NN1731.  Catalyst intends to file an Investigational New Drug (IND) application for CB 813 and commence human clinical studies in hemophilia patients in 2009.

“Patients receiving factor VIIa therapy typically require multiple doses of drug and extended treatment times to manage acute bleeding episodes,” said Gilbert C. White, II, M.D., Executive Vice President for Research and Director, Blood Research Institute, Blood Center of Wisconsin.  “I am excited by the potential shown by CB 813 to improve the efficiency of treatment by achieving a faster onset and an extended duration of therapeutic benefit to patients.  These advantages may significantly reduce both the number of doses and treatment time required to manage bleeding and restore normal hemostasis.”

“The selection of this development candidate is an exciting milestone for Catalyst.  Created to more rapidly and effectively stop bleeding, CB 813 holds great promise as a new agent in the treatment of hemophilia,” said Nassim Usman, Ph.D., Chief Executive Officer of Catalyst Biosciences.  “CB 813 is representative of a new class of improved variants of factor VIIa discovered using Catalyst’s proprietary protease engineering platform and know-how.  Created within 12 months from program start up, CB 813 and related compounds demonstrate the power of our platform.”

About Hemophilia

Hemophilia is a rare but serious bleeding disorder that results from a genetic or an acquired deficiency of a protein required for normal blood coagulation, such as
factor VIII (hemophilia A) or factor IX (hemophilia B) in addition to factor VIIa for refractory hemophilia A and hemophilia B patients.  Hemophilia patients suffer from potentially life-threatening spontaneous bleeding episodes and substantially prolonged bleeding times upon injury.  In cases of severe hemophilia, spontaneous bleeding into muscles or joints is frequent and often results in permanent, disabling joint damage.

Hemophilia patients are initially treated by replacement therapy, receiving purified versions of the clotting factor in which they are deficient.  However, a significant number of hemophilia A patients develop inhibitory antibodies against factor VIII and therefore become refractory to this treatment.  These patients are then treated with factor VIIa, a protease enzyme that can both initiate blood clotting and, at high doses, “bypass” the factor VIII-dependent step in coagulation.  Hemophilia B patients refractory to factor IX may also be treated with factor VIIa.

About Catalyst Biosciences

Catalyst Biosciences is developing the next generation of biopharmaceuticals by harnessing the catalytic power of engineered proteases to target proteins underlying disease.  Catalyst’s discovery platform rapidly creates and optimizes tailor-made protease drug candidates, called Alterase™ therapeutics, that are applicable across a broad spectrum of disease categories. Initially, Catalyst is focusing its product development efforts on Alterase drug candidates for hemophilia, age-related macular degeneration, oncology and inflammation.  To date, Catalyst has established two discovery research and product development agreements with Wyeth Pharmaceuticals and Centocor Research & Development, Inc.  Catalyst is privately-held with backing by leading venture firms including Burrill & Company, HealthCare Ventures, Johnson & Johnson Development Corporation, Morgenthaler Ventures, Novartis BioVentures, RCT BioVentures and Sofinnova Ventures.  For more information, please visit www.catbio.com.

Catalyst Biosciences, Inc.

Erik Wiberg

+1.650.266.8654

ewiberg@catbio.com

BCC Partners for Catalyst Biosciences, Inc.

Karen L. Bergman or Michelle Corral

+1.650.575.1509 or +1.415.794.8662