Schematic representation of the major mammalian pattern recognition receptor families. Common to all Toll-like receptors (TLRs) are leucine-rich repeats and a Toll-interleukin 1 receptor (TIR) domain. Nucleotide-binding oligomerization (NOD)-like receptors (NLRs) are characterised by a central NOD domain and the defining feature of RIG-I-like receptors (RLRs) is a helicase domain. The two members of the recently described AIM2-like receptors (ALRs) have HIN domains in common. The C-type lectin receptors (CLRs) are a very diverse group of proteins with no common domain structure. The type I (DEC-205) and type II (DC-SIGN) transmembrane CLRs are represented here.