Intracellular vs extracellular recognition of pathogens--common concepts in mammals and flies.

The existence of mammalian Toll-like receptors (TLRs) and the recent identification of two mammalian nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins (NOD1 and NOD2) as intracellular sensors of bacterial products bring new insights into the possibility of extracellular versus intrACEllular pathogen recognition and signal transduction depending on the nature of the infection.