The GEO database, known as GENE EXPRESSION OMNIBUS, is a gene expression database created and maintained by NCBI, the National Center for Biotechnology Information. It was created in 2000 and contains high-throughput gene expression data submitted by research institutions around the world, which means that as long as the paper is currently published, the data for the gene expression tests involved in the paper can be found in this database. This database is supposed to be the database for introductory bioinformatics learning to mine, the volume of postings is estimated to be in the thousands per year, and GEO has a wealth of sequencing files on it, oncology, non-oncology, etc. Almost everything is available and can be mined for free. There is a lot of information about this database online, so I won't go into it. For those interested, take a look above the raw letter skill tree.