Rx in potato encodes a protein with a nucleotide binding site (NBS) and leucine-rich repeats (LRR) that confers resistance against Potato virus X. The NBS and LRR domains in Rx are present in many disease resistance proteins in plants and in regulators of apoptosis in animals. To investigate structure-function relationships of NBS-LRR proteins we exploited the potential of Rx to mediate a cell death response. With wild-type Rx cell death is elicited only in the presence of the viral coat protein. However, following random mutagenesis of Rx, we identified mutants in which cell death is activated in the absence of viral coat protein. Out of 2500 Rx clones tested there were seven constitutive gain-of-function mutants carrying eight independent mutations. The mutations encoded changes in the LRR or in conserved RNBS-D and MHD motifs of the NBS. Based on these findings we propose that there are inhibitory domains in the NBS and LRR. The constitutive gain-of-function phenotypes would be due to deletion or modification of these inhibitory domains. However activation of Rx is not simply release of negative regulation by the LRR and adjacent sequence because deleted forms of Rx that lack constitutive gain of function mutations are not active unless the protein is overexpressed.