Arrowhead Scheduled to Be Listed on Russell 3000 Index

Arrowhead Research Corporation (NASDAQ:ARWR) announced today that it is scheduled to join the Russell 3000TM Index when the Russell Investment Group reconstitutes its family of U.S. indexes on June 22, 2007, according to a preliminary membership list posted on Monday, June 11, 2007, on www.russell.com. Arrowhead is already a member of the Russell Microcap Index. "Arrowhead joined the Russell Microcap Index just one year ago. We are pleased to have achieved inclusion in the more exclusive Russell 3000 Index as well,” said R. Bruce Stewart, Chairman and CEO of Arrowhead. "The growth that qualified Arrowhead for Russell 3000 was made possible by the vision and hard work of the talented people at Arrowhead and our majority-owned subsidiaries.” Russell, a global leader in multi-manager investment services, provides investment products and services in 44 countries. Russell manages more than $167 billion in assets and advises clients worldwide representing $2.4 trillion. Russell indexes are widely used by investment managers and institutional investors for index funds and as benchmarks for both passive and active investment strategies. An industry-leading $4 trillion in assets currently are evaluated and benchmarked against these Russell indexes. About Arrowhead Research Corporation Arrowhead Research Corporation (www.arrowheadresearch.com) (NASDAQ:ARWR) is a leading nanotechnology company commercializing new technologies in the areas of life sciences, electronics, and energy. Arrowhead is building value for shareholders through the progress of majority-owned subsidiaries founded on nanotechnologies originally developed at universities. The company works closely with universities to source early stage deals and to generate rights to intellectual property covering promising new nanotechnologies. Currently, Arrowhead has four subsidiaries commercializing nanotech products and applications, including anti-cancer drugs, RNAi therapeutics, carbon-based electronics, and compound semiconductor materials. Safe Harbor Statement under the Private Securities Litigation Reform Act of 1995: This news release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995. These statements are based upon our current expectations and speak only as of the date hereof. Our actual results may differ materially and adversely from those expressed in any forward-looking statements as a result of various factors and uncertainties, including the future success of our scientific studies, our ability to successfully develop products, rapid technological change in our markets, changes in demand for our future products, legislative, regulatory and competitive developments and general economic conditions. Our latest Annual Report on Form 10-K, recent and forthcoming Quarterly Reports on Form 10-Q, recent Current Reports on Form 8-K, our Registration Statements on Form S-3, and other SEC filings discuss some of the important risk factors that may affect our business, results of operations and financial condition. We undertake no obligation to revise or update publicly any forward-looking statements for any reason.