induces a different feeding structure named syncytium. While Mi resistance results in localized tissue necrosis, no such hypersensitive reaction is observed with Hs1. As judged from sequence analysis, Hs1 is also is part of a signal cascade, but seems to be located in the plasma membrane whereas Mi is cytoplasmic. These findings have two major implications for future breeding strategies and on our understanding of host–pathogen interactions. At first glance, it seems puzzling that one gene causes resistance to species that obviously have nothing in common except that both make use of their stylets to feed from the host tissue. However, the aphid inserts its stylet intercellularly to feed from the phloem, whereas the nematode penetrates the cell membrane with its stylet to take up nutrients from inside the cell. As it is extremely unlikely that the Mi protein reacts with the same ligand on nematodes and aphids, it cannot be the primary receptor, and probably partakes in an intracellular signal transduction cascade that starts with ligand binding to yet unknown receptors and ends with the activation of defenserelated genes within the nucleus that ultimately results in localized cell death. The prospects for breeding new disease resistant tomato varieties are immediate, and it is tempting to think of transforming unrelated crop species that are attacked by aphids and root knot nematodes. Some caveats, however, would urge caution in the implementation of this attractive strategy. It seems unlikely that Mi will be effective against cyst nematodes because of the lack of homology to the Hs1 gene. Also, breeders must be aware of resistance breaking aphids frequently evolving from the huge number of individuals in the field. One such isolate has already been shown to break the Mi resistance in Motelle and transgenics. As Mi resistance follows a one-to-one relationship, in which a virulence gene of the pathogen matches a resistance gene from the host single, mutational events can result in virulent isolates that pose a threat to all varieties.