 Dear students, in this module, I'm going to talk about the types of BLAST software that are available online. So to begin with, BLAST can help you to find sequences that are known or unknown from protein or nucleotide databases. So this is very useful because if you have a nucleotide sequence that is unknown till date, then you can search for similar nucleotide sequences in the database. Or if you obtain a novel protein sequence, then you can take this sequence, BLAST it against the protein sequence databases and get similar proteins or peptides from the protein databases. And this can be done very fast by BLAST and you can process hundreds and thousands of sequences in a very short amount of time. So to begin with, as you already know, there are two general types of BLAST. So the first one is the nucleotide BLAST and secondly the protein BLAST. So for the nucleotide BLAST or simply BLAST N, you compare a nucleotide query versus a nucleotide database. So the query is nucleotide and the DB or the database is nucleotide as well. The second type is the BLAST B or protein BLAST. So what it does is it compares an amino acid sequence with an amino acid database or a protein database. So your query is a amino acid sequence and your database is proteins or peptides. So in this way you can compare protein sequences with protein databases or nucleotide sequences with nucleotide databases. However, what if you have a nucleotide sequence and you want to compare it with a protein sequence database. That is you want to see if your nucleotide sequence can code for a protein which is already reported in the database. So let's see how that can be done. So here you go, you have another version of BLAST that is the BLAST X. So what it does is it compares a nucleotide sequence against a protein sequence database. So your query is a nucleotide sequence and your database is proteins. So this is a very interesting BLAST because to create a protein from a nucleotide sequence you need to create all six open reading frames. Remember a nucleotide sequence can code for six different ORFs and the longest ORF can form a protein. So therefore if you have a BLAST X then you have to create six different ORFs for the nucleotide sequence and then translate into a protein before comparing it to the protein database. Also the BLAST X it helps you to find the potential translation products. So if there are some proteins that are not reported in the protein database but you have a gene sequence that is coding for proteins then it can help you to find novel proteins as well. So next we have the T BLAST N. So this is essentially a reverse of BLAST X that we just saw in the previous slide. So T BLAST N compares a protein sequence versus a nucleotide database. So your query is protein sequence that is a minoacid sequence and your database is a nucleotide database. So how do you do that? So what you do is you translate the nucleotide sequences within the database into proteins and then compare it with the query sequence. Next you have the T BLAST X. So it compares the six frame that is the six open reading frames that were translated from a nucleotide query against a six frame translated protein of a nucleotide database. So your query is nucleotide and your database is also a nucleotide database. So all you are doing is you are creating the six ORFs and then comparing them with the six ORFs from the database. So each one ORF is compared against the all six possibilities and then the BLAST is completed. So of course this involves a lot more computation as compared to a simple BLAST N or a BLAST B. So in conclusion BLAST helps you to quickly align and compare nucleotide and a minoacid sequences and you can create different combinations of these comparisons in order to compare genes with proteins and proteins with genes or genes with genes by looking at the different ORFs that can be coded. So there are several types of BLAST which are all available online free of cost at the NCBI website.