Tomato Cf genes confer resistance to the leaf mould pathogen Cladosporium fulvum through recognition of distinct fungal-encoded avirulence (Avr) peptides secreted into the leaf apoplast during infection. These Avr determinants are small cysteine-rich proteins that exhibit no primary sequence homology (Luderer and Joosten, 2001). The tomato–C. fulvum interaction is an attractive model system to study the molecular basis of recognition specificity in plant disease resistance (R) proteins, and R-protein mediated defence responses. Cf proteins are type I transmembrane glycoproteins comprised predominantly of extracytoplasmic leucine-rich repeats (LRRs), a membrane-spanning region, and a short cytoplasmic domain that lacks an obvious signalling function (reviewed in Thomas et al., 1998). These R proteins activate a hypersensitive response (HR) upon recognition of their cognate Avr proteins.