The leucine-rich (LRR) repeat is a 20-30 amino acid motif that forms a hydrophobic a/∫ horseshoe fold, allowing it to accommodate several leucine residues within a tightly packed core. All LRR repeats contain a variable segment and a highly conserved segment, the latter of which accounts for 11 or 12 residues of the entire LRR motif. NOD9, also known as NLRX1, NOD26 or NOD5, is a 975 amino acid outer mitochondrial membrane protein that contains one NACHT domain and four LRR repeats. Expressed at high levels in heart, muscle and mammary gland, NOD9 plays a role in antiviral signaling, specifically via inhibition of virus-induced helicases, thereby acting as a negative regulator of antiviral responses. Two isoforms of NOD9 exist due to alternative splicing events.