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RIG-I-like receptors, abbreviated RLRs, are a type of intracellular pattern recognition receptor involved in the recognition of viruses by the innate immune system. There are three RLRs: RIG-I, MDA5, and LGP2 that act as sensors of viral replication within the cytoplasm of human cells. RLRs detect viral replication through direct interaction with dsRNA, which is produced by RNA viruses to form their genome (dsRNA viruses) or as a part of their replication cycle. Two of the RLRs, RIG-I and MDA5, possess the ability to induce a cellular response (via CARD domains) upon recognition of viral dsRNA. LGP2, the remaining RLR, lacks the ability to induce signaling alone (due to the absence of CARD domains), but has been found to be necessary for effective RIG-I and MDA5-mediated antiviral responses. Recognition of viruses by RLRs typically leads to the production of type I interferons through the activation of a complex signaling network.
The other two families of PRRs, the NOD-like receptors (NLRs) and the RIG-like helicases (RLHs) are soluble receptors present in the cytosol and act as sensors to detect a variety of viral and bacterial products.