NCBI BLAST is like google for bioinformatics. It helps in quickly comparing any nucleotide or protein sequence with various databases stored at NCBI.

Database option

# Often used databases
1. Human genome + transcriptome - relevant for blastn
2. Mouse genome + transcriptome - relevant for blastn
3. NR/NT - non-redundant set of nucleotides - relevant for blastn
4. Assembled genomes - relevant for blastn
5. NR - non-redundant set of proteins - relevant for blastp

Organism option

 # You can include or exclude a class of organisms.

References

Nucleotide BLAST

https://blast.ncbi.nlm.nih.gov/Blast.cgi?PAGE_TYPE=BlastSearch

Protein BLAST

https://blast.ncbi.nlm.nih.gov/Blast.cgi?PAGE=Proteins

BLASTX

https://blast.ncbi.nlm.nih.gov/Blast.cgi?PROGRAM=blastx&PAGE_TYPE=BlastSearch

TBLASTN

https://blast.ncbi.nlm.nih.gov/Blast.cgi?PROGRAM=tblastn&PAGE_TYPE=BlastSearch

Microbial BLAST

https://blast.ncbi.nlm.nih.gov/Blast.cgi?PAGE_TYPE=BlastSearch&BLAST_SPEC=MicrobialGenomes