Anti-viral innate immune responses are triggered by pathogen-associated molecular patters (such as the accumulation of intracellular nucleic acids resulting from virus infections) and represent the first line of defense against numerous infectious organisms. The Toll-like receptors (TLR) 3, 7, 8, and 9 are expressed in immune cells and function as transmembrane pattern recognition receptors to detect foreign nucleic acids. Other proteins that play similar roles, such as RIG-1 and MDA5 are members of a CARD-helicase family and are expressed in the cytoplasm. A third protein, LGP2, is similar to RIG-1 and MDA5, except for lacking a homologous CARD domain. It is thought to act as an element of negative-feedback regulation of intracellular antiviral signaling. At least four isoforms of LGP2 are known to exist.