Blast e-value 0E0

Question

[darzenis]

I performed blast searches (with an e-value cut-off of 1E-5) on de novo assembled transcripts against the ncbi nr database. Many transcripts received hits with a reported e-value of 0E0. I imagine that an e-value of 0 is almost impossible (similar to reporting a p-value of 0), so does this mean that the e-value was so low it was just rounded to 0? Or is it some kind of error message?

[sunnykevin97]

It means exact match.

Answer 1

[Andrzej Zielezinski]

Very low e-values are rounded to zero. BLAST uses double float point representation, so e-values lower than approximately 5*10^−324 are rounded to 0.