In 2009, the Sequence Read Archive (SRA, see below) (10), a repository for data generated by next-generation sequencing technologies, was added to the Entrez system of databases, thereby allowing the SRA data to be searched using fielded text queries and more easily linked with related data at NCBI. Within Entrez SRA (www.ncbi.nlm.nih.gov/sra/), the data are organized into four types of records: studies (SRP accessions), experiments (SRX accessions), samples (SRS accessions) and runs (SRR accessions). Studies contain one or more experiments, each of which contains one or more runs, each of which in turn may contain data on tens of millions of individual reads. The various record types representing data from a study are all linked to one another within Entrez, allowing users to browse the data easily on the web.
