Human Genome U133A 2.0 Array This GeneChip array contains 22,500 probe sets representing greater than 18,000 transcripts derived from approximately 14,500 well-substantiated human genes. All probe sets and normalization genes on the HG-U133A Array are identically represented on this array. Sequences used in the design were selected from GenBank, dbEST, and RefSeq. Sequence clusters were created from Build 133 of UniGene (April 20, 2001) and refined by analysis and comparison with a number of other publicly available databases including the Washington University EST trace repository and the University of California, Santa Cruz Golden-Path human genome database (April 2001 release). Human Genome U133 Set This two-array set of GeneChip probe arrays contains almost 45,000 probe sets representing greater than 39,000 transcripts derived from approximately 33,000 well-substantiated human genes. Sequences used in the design of the set were selected from GenBank, dbEST, and RefSeq. Sequence clusters were created from Build 133 of UniGene (April 20, 2001) and refined by analysis and comparison with a number of other publicly avail- able databases, including the Washington University EST trace repository and the University of California, Santa Cruz Golden-Path human genome database (April 2001 release). The HG-U133A Array includes representation of the RefSeq database sequences, as well as probe sets that are related to sequences represented previously on the Human Genome U95Av2 Array. The HG-U133B Array primarily contains probe sets representing EST clusters.