Blast 2.28 evalue threshold for saving hits

Question

[xi100f]

Hi,

I am trying to limit number of saved hits to only those with evalues <1e-180.

According to help file of blast I use ( BLAST 2.2.28+) one of the general serch options is

-evalue <Real>
   Expectation value (E) threshold for saving hits 
   Default = `10'

but

blastn -db blast.db -query queryfile -outfmt 6  -evalue 1e-180 > blast.out

returns only hits with evalue 0.0

while

blastn -db blast.db -query queryfile -outfmt 6  > blast.out

returns plenty of hits including those with evalues greater or lesser than 1e-180.

Does anyone knows what I am doing wrong?

Thanks in advance!

X

[Jean-Karim Heriche]

It could be due to numerical precision, i.e. at some point 1e-180 = 0.