Central to plant oomycete pathosystems is a complex signaling process in which multiple effector proteins are delivered either into the host cell or to the free diffusional space outside the plasma membrane to manipulate host cell structure and function. The effector proteins can either promote AL 082D06 infection, resulting in benefit to the pathogen, or trigger defensive responses that preclude multiplication of the pathogen. In view of their importance, there is considerable interest in the discovery and characterization of the proteins mediating the host�Cpathogen interaction. Various classes of effector genes have already been characterized for Aristolochic-acid-A oomycetes, including the RxLR family, which currently comprises hundreds of candidate genes. A second class of effectors, the CRN proteins, first identified through an in planta functional expression assay, includes a complex family of relatively large proteins. Finally, there are several apoplastic effectors classified as enzyme inhibitors involved in protection against host defense responses. Schornack et al. recently reviewed different aspects of the oomycete effectors. The effector secretome of Phytophthora is now known to be much more complex than initially expected and is starting to be completely understood thanks to all the progress made during the past few years in this field. Data mining is one stage in a long�Cterm process of discovery that can be used as a powerful tool to evaluate existing information depending on the researcher��s goal. To date, a considerable number of sequences have been obtained from cDNA libraries from P. infestans – infected host plants during compatible and incompatible interactions. Some of these sequences encode effector proteins expressed by the pathogen during infection. In previous studies, sequence origin in P. infestans�Cchallenged libraries has mainly been analyzed by GC content and/or by sequence similarity. These methodologies lack accuracy because they may overlook sequences belonging to the studied organism or having different GC percentages. The draft of the whole genome of P. infestans is now available, making it possible to analyze sequence origins precisely within a large data set using bioinformatics tools.