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