Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The NOD-like receptors (NLRs) are cytoplasmic proteins that may have a variety of functions in regulation of inflammatory and apoptotic responses; the official definition of NLR is currently "Nucleotide-binding domain, Leucine-Rich repeat containing" protein since these proteins are composed of conserved "modules" including a central nucleotide-binding oligomerization domain and a series of tandem leucine-rich repeats . NLRs are encoded by genes from a large gene family present in many different animal species; there are more than 20 NLR genes in humans. Many are thought to serve as pattern recognition receptors (PRRs) which sense microbial products in the cytoplasm of cells, although some members have different functions.