ArabiTag on-line tool for assessing frequency of splicing choices from Arabidopsis gene models. Screen captures showing query and results pages from ArabiTag http://www.transvar.org/arabitag are shown. Searching for locus id AT2G39730, encoding rubisco activase, a well-known example of conserved AS in plants [14, 34], retrieves three AS Event ids and their corresponding Difference Regions. The alternative splicing choices for the third AS event on the list are supported by multiple ESTs. Clicking the AS Event id link opens an AS Report page in ArabiTag that shows the breakdown of support for each choice, by library, in three different tables according to which choice the ESTs support. Libraries that contain a mix of different ESTs that support both choices appear in the first table, libraries with ESTs that support just one alternative (GA) appear in the next table, and libraries with ESTs that support the other alternative (GP) appear in the final table. In all cases, the number of ESTs from each library that support GA or GP are also listed, thus providing the user with information about possible tissue or sample-type specific alternative splicing. However, in this case, both rubisco activase forms are widely expressed, suggesting they may co-occur in the same cells or cell types.