Resistance in tomato against the fungal pathogen Alternaria alternata f. sp. lycopersicon is conferred by a single semi dominant locus called Asc . The locus was positioned between two closely linked Restriction Fragment Length Polymorphism (RFLP) markers, thus allowing to clone the Asc gene(s) by a map based strategy. A contig of Yeast Artificial Chromosomes (YACs) was obtained, providing closer markers around the Asc locus. The position of the Asc locus was delimited to a 200 kb fragment. As a first step, the Asc gene(s) will be cloned into cosmids obtained from the YAC containing the Asc locus. The corresponding genomic and the cDNA fragment(s) will be sequenced, giving access to the structure of the gene(s). As a second step, by performing experiments on gene expression and regulation and by studying gene homologues in other species, the Asc gene(s) function will be determined in relation to the disease resistance.