The GeneChip Arabidopsis ATH1 probe array, designed in collaboration with TIGR, contains more than 22,500 probe sets representing approximately 24,000 genes on a single array. The array is based on information from the international Arabidopsis sequencing project that was formally completed in December2000. In parallel and subsequent to the genome’s completion, TIGR reannotated the entire genome in a project funded by the National Science Foundation and the resulting data were used in the design of this array. Data from the TIGR database (ATH1-121501) are hosted on the NetAffx Analysis Center.