DAS is a specification of a protocol for requesting and returning annotation data for genomic regions. DAS allows sequence annotation to be stored in a decentralised manner, by multiple third-party annotators, and integrated on an as-needed basis by client-side software.
A single DAS server is designated as either an annotation server or as the reference server. The reference server provides essential structural information about the genome: the physical map which relates one entry point to another (where an "entry point" is an arbitrary segment of the sequence, such as a sequenced BAC, a contig, or a whole assembled chromosome), the DNA sequence for each entry point, and some standard authorship information. Using either a freestanding application or a web site, such as Ensembl, that acts like a DAS client, researchers can interrogate one or more annotation servers to retrieve features in a region of interest. The servers return the results using a standard data format, allowing the sequence browser to integrate the annotations and display them in graphical or tabular form. No attempt is made to automatically resolve contradictions between different third-party annotations. Indeed, the ability to make comparisons between annotations from different centres is one of the main uses for DAS.
A DAS request consists of a specially formatted URL in a simple HTTP request (as used by any web browser), and all DAS data are returned in an XML document. The XML document format which is used is derived from GFF.
The Ensembl project have extended the use of the DAS specification to not only cover genomic annotations, but to also allow for proteomic annotations and the annotation of non-positional features. See the text on DAS in Ensembl for more info about this.
The Distributed Annotation System was originally developed by Lincoln D. Stein, Sean Eddy and Robin Dowell in 2000. The DAS home page is at http://www.biodas.org/.
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