<p>Megablast is designed to find very similar sequences. It is highly specific but has a very low sensitivity. This is because it has a very large <a href="https://www.metagenomics.wiki/tools/blast/default-word-size" rel="nofollow noreferrer">word size</a>. By default, I think it will use a <a href="https://www.ncbi.nlm.nih.gov/books/NBK279684/table/appendices.T.blastn_application_options/" rel="nofollow noreferrer">word size of 28 nt</a>. That's almost your entire sequence, so if your target sequence doesn't have 28 contiguous nucleotides that are <em>identical</em> to 28 contiguous nucleotides of your query, no hit will be found.</p>
<p>If you are looking for distant homologous sequences, you should use simple <code>blastn</code> which defaults to a word size of 11. That will still likely be too small, you might want to change it to something lower (your blast tool should have an option for that), but it will give you more results than megablast.</p>
<p>Finally, if these 32nt are translated into protein, you can consider using tblastn instead.</p>

CC BY-SA 4.0
2023-10-28T11:25:33.853Z
2023-10-28T11:25:33.853Z
terdon
<p>I have a 32-bp query that I want to perform BLAST+. my query represents different RNA viruses and I do expect that my hits get more than 500,000.
but in practice, when I perform it I get around only 700 hits. would you please kindly help me how to increase the efficacy of my BLAST+?
I leave my syntax here.</p>
<pre><code>blastn -task megablast -db nr -outfmt 7 -remote -max_target_seqs 99999999 -perc_identity 80 -query D:/Query.fas  -out D:/result.txt 
</code></pre>

CC BY-SA 4.0
2023-10-26T12:49:10.193Z
Can you explain why you expect many queries? More importantly, what is the biological question you are trying to answer here? Are you looking for distant homologs? Close homologs? Why did you choose to use megablast? That's an odd choice for such a short sequence.
terdon
CC BY-SA 4.0
2023-10-27T19:53:47.217Z
Thanks for your comment.
I expect many hits because there are many hits for my query as I know my query is common among RNA viruses, it is such a conservative seq. 
Yes, I want to extract all sequences in the NR database that are similar to my query.
I think megablst was a bad choice, because when I choose -task blastn my hits were significantly improved (+500,000), but still I encounter an error by blast+ stating "search cannot be formatted" resulting in finding just some of those hits.
Farzad Beikpour
CC BY-SA 4.0
2023-10-27T21:04:12.463Z
Please [edit] your question and explain this. Megablast is only useful for very similar sequences. And long ones, usually. It doesn't really make sense to use it for a 32bp query, that will be smaller than the smallest thing it can find.  So please [edit] and explain what you are trying to do. You don't want similar sequences, presumably you are looking for homologs, right? I mean explain the biological question you are trying to answer, so we know how to help.
terdon
<blast><blastn>
how to use BLAST+ in an optimal way
# how to use BLAST+ in an optimal way

(Asked by: username_0 on 2023-10-26)

I have a 32-bp query that I want to perform BLAST+. my query represents different RNA viruses and I do expect that my hits get more than 500,000.
but in practice, when I perform it I get around only 700 hits. would you please kindly help me how to increase the efficacy of my BLAST+?
I leave my syntax here.
blastn -task megablast -db nr -outfmt 7 -remote -max_target_seqs 99999999 -perc_identity 80 -query D:/Query.fas  -out D:/result.txt

**Tags:** blast, blastn
> **Comments:**
> **username_1**: Can you explain why you expect many queries? More importantly, what is the biological question you are trying to answer here? Are you looking for distant homologs? Close homologs? Why did you choose to use megablast? That's an odd choice for such a short sequence.  
> **username_0**: Thanks for your comment. I expect many hits because there are many hits for my query as I know my query is common among RNA viruses, it is such a conservative seq.  Yes, I want to extract all sequences in the NR database that are similar to my query. I think megablst was a bad choice, because when I choose -task blastn my hits were significantly improved (+500,000), but still I encounter an error by blast+ stating "search cannot be formatted" resulting in finding just some of those hits.  
> **username_1**: Please [edit] your question and explain this. Megablast is only useful for very similar sequences. And long ones, usually. It doesn't really make sense to use it for a 32bp query, that will be smaller than the smallest thing it can find.  So please [edit] and explain what you are trying to do. You don't want similar sequences, presumably you are looking for homologs, right? I mean explain the biological question you are trying to answer, so we know how to help.  

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## 1 answers

### Answer by username_1 on 2023-10-28. Score: 1

Megablast is designed to find very similar sequences. It is highly specific but has a very low sensitivity. This is because it has a very large [word size](https://www.metagenomics.wiki/tools/blast/default-word-size). By default, I think it will use a [word size of 28 nt](https://www.ncbi.nlm.nih.gov/books/NBK279684/table/appendices.T.blastn_application_options/). That's almost your entire sequence, so if your target sequence doesn't have 28 contiguous nucleotides that are *identical* to 28 contiguous nucleotides of your query, no hit will be found.

If you are looking for distant homologous sequences, you should use simple `blastn` which defaults to a word size of 11. That will still likely be too small, you might want to change it to something lower (your blast tool should have an option for that), but it will give you more results than megablast.

Finally, if these 32nt are translated into protein, you can consider using tblastn instead.


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