Acceleron Pharma Inc. (NASDAQ:XLRN), a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases, has been selected for addition to the Russell 2000 ®, Russell 3000 ® and Russell Global ® Indexes as part of Russell Investments’ quarterly addition of select initial public offering (IPO) companies. Acceleron will join these indexes after the NASDAQ market closes on December 20, 2013. Membership in the Russell 2000 Index results in automatic inclusion in the appropriate growth and value style indexes. The Russell 2000 Index measures the performance of the small-cap segment of the market and is a subset of the Russell 3000, which also serves as the U.S. component to the Russell Global Index. Russell Investments determines membership for its equity indexes primarily by objective market-capitalization rankings and style attributes. Russell indexes are widely used by investment managers and institutional investors for index funds and as benchmarks for both passive and active investment strategies, and approximately $4.1 trillion in assets are benchmarked to Russell Indexes. About Acceleron Acceleron is a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. The company is a leader in understanding the biology of the TGF-β protein superfamily, a large and diverse group of molecules that are key regulators in the growth and repair of tissues throughout the human body, and in targeting these pathways to develop important new medicines. Acceleron has built a highly productive R&D platform that has generated innovative clinical and preclinical protein therapeutic candidates with novel mechanisms of action. These protein therapeutic candidates have the potential to significantly improve clinical outcomes for patients with cancer and rare diseases. For more information, please visit www.acceleronpharma.com.