The Meaning Of The E In Blast Output
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Entering edit mode
10.2 years ago
David M ▴ 560

I have a question that's been floating around in my head for a while, and a colleague recently asked the same thing so I thought I should finally get to the bottom of it.

What does the 'e' in the output of a BLAST search refer to? Specifically, does the symbol 'e' refer to 'x10^' or does it refer to the mathematical constant 'e=2.72...'? I could see a justification for either: e as 'x10^' is a convenient notation, while 'e=2.72...' is actually used in the calculation of the e-value score itself.

This hasn't really mattered in assessing the relative quality of a hit, or determining the homology between sequences, but I thought I'd better find an answer for this question.

Thanks!

David

blast • 2.3k views
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Entering edit mode
10.2 years ago

The "e" stands for "10^".

This example output has an e-value of "5.0e-120".

In this context, it would make no sense for e to be the Euler number, as that would mean: 5 * 2.72 * (-120), which is -1632, but e-values can't be smaller than 0!

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Entering edit mode

Thanks for the answer! When I mentioned "e as 2.72...", I was actually referring to 5.0e^(-120), using your example.