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When the Expect value is increased from the default value of 10, a larger list with more low-scoring hits can be reported. You can change the Expect value threshold on most BLAST search pages. The Expect value can also be used as a convenient way to create a significance threshold for reporting results. For more details please see the calculations in the BLAST Course. These high E values make sense because shorter sequences have a higher probability of occurring in the database purely by chance. This is because the calculation of the E value takes into account the length of the query sequence. However, keep in mind that virtually identical short alignments have relatively high E values. The lower the E-value, or the closer it is to zero, the more "significant" the match is. For example, an E value of 1 assigned to a hit can be interpreted as meaning that in a database of the current size one might expect to see 1 match with a similar score simply by chance. Essentially, the E value describes the random background noise. It decreases exponentially as the Score (S) of the match increases. The Expect value (E) is a parameter that describes the number of hits one can "expect" to see by chance when searching a database of a particular size. Q: How to use BLAST to align two sequences without a database search. Searches will be run at lower priority than interactive searches This service uses NCBI compute resources and is considered a batch search. 4.) Submit searches through the NCBI URL API.Should be run with stand-alone BLAST or through an instance at a cloud provider. Searches run at off-peak hours may have better throughput. Searches will be run at lower priority than interactive searchesįrom the NCBI BLAST web pages. This client uses NCBI compute resources and is considered a batch search. The stand-alone executables can send searches to the BLAST server using the -remote flag. See theīLAST searches at a Cloud Provider page for details. A simple BLAST web page is also included. This AMI provides access to the stand-alone BLASTĮxectuables, but also has a network API similar to the URL API provided by at the NCBI. The NCBI has an Amazon Machine Image (AMI) at Amazon Web Services. A manual is available for stand-alone BLAST here.