RNA can serve as a danger signal, both in its double-stranded form (that is associated with viral infection), as well as single-stranded RNA (ssRNA). Specifically, guanosine (G)- and uridine (U)-rich ssRNA oligonucleotides derived from human immunodeficiency virus-1 (HIV-1), for example, stimulate dendritic cells (DC) and macrophages to secrete interferon-alpha and proinflammatory, as well as regulatory, cytokines. This has been found to be mediated by TLR7, as well as TLR8. Separate studies showed that imidazoquinoline compounds (e.g. imiquimod and R-848, low-molecular-weight immune response modifiers that can induce the synthesis of interferon-alpha) also exert their effects in a MyD88-dependent fashion independently through TLR7 and 8 (Heil et al. 2004).