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The Expect value (E) is a parameter that describes the number of hits
one can "expect" to see just by chance when searching a database of a
particular size. It decreases exponentially with the Score (S) that is
assigned to a match between two sequences. Essentially, the E value
describes the random background noise that exists for matches between
sequences. 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. This means that the lower the E-value, or the closer it is to
"0" the more "significant" the match is. (For more details see the
calculations in NCBI's BLAST Course.) Use the sequence from
Exercise 1 again:
- Perform a blastx search for Homo sapiens seuqences against
- Look for a hit that appears in
both outputs and compare the
E-values: do they change? Why?
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