T he infection of a cell by a virus produces a mixture of viable and noninfectious progeny particles (1-3). A common class of noninfectious particles has defective genomes, often carrying deletions in essential genes that disable their ability to productively infect cells. However, in coinfections with viable or helper virus, the genomes of these defective particles compete with the viral replication machinery and packaging processes, interfering with infectious virus production in vitro (4, 5), and often reducing virulence in vivo (6, 7). These so-called defective interfering particles (DIPs) have for many decades been observed in laboratory cultures of virtually every class of DNA and RNA virus (4,8). More recently, DIPs have been isolated and characterized from patients infected with influenza virus (9), individuals infected with dengue virus (10, 11), and birds infected with West Nile virus (12). Moreover, sequencing of patient and natural isolates has contributed to a growing list of diverse viral genomes that carry deletions in essential genes or regulatory sequences, including hepatitis C virus (HCV) (13), polyomavirus BK (14), hepatitis B virus (15), human papillomavirus type 16 (16), and baculovirus (17). Notably, for hepatitis C virus, trans-complementation studies showed that defective HCV genomes could be encapsidated and released from cells as infectious virus-like particles (13)