Contains a central nucleotide-binding (NB) … Contains a central nucleotide-binding (NB) subdomain as part of a larger entity called the NB-ARC domain. C-terminal to the NB-ARC domain lies a leucine-rich repeat (LRR) domain, which is sometimes followed by an extension of variable length. Hence, this group of R proteins is collectively referred to as NB-LRR proteins. If N-terminal region shows homology to the protein domain Interleukin-1 Receptor (IL-1R), called the TIR domain, these proteins are referred to as TIR-NB-LRR or TNL. eins are referred to as TIR-NB-LRR or TNL.