A BLAST is a Basic Local Alignment Search Tool. This tool can be used to compare sequences from a genome with sequences in other organisms. To use the tool, go to http://blast.ncbi.nlm.nih.gov/Blast.cgi.
It is a very effective method for comparing DNA sequences, but you need to go to the website with some sequence in mind. You can only compare some piece that you know, rather than being able to search and find a piece of DNA sequence with which to work.
Once you input your DNA sequence, you can see what other organisms in the database have similar sequences. Depending on the size and nature of your sequence, you may find close relatives or you may not.
Applying this to cancer research, if you know a sequence of DNA that is mutated to cause cancer in someone, you can potentially find an organism with a similar sequence of DNA and compare how that organism is able to handle mutations to that segment of DNA.
One pro of the technique is it being quick, especially considering how large the database it has to run the segment through. Some sequences may take longer than others and people might be impatient with the amount of time it takes, but given what it is accomplishing, it is timely. BLAST Comparison also gives you the opportunity to compare multiple species to your target DNA strand, rather than selecting a single species with which to compare it.
One con potential con, as addressed above is that you need to know what you are looking for. Typing in any of the sequence wrong can lead to no or misleading results. Sometimes the program also does not run. Switching computers has helped me when I have run into this problem myself.
The Human Genome Project and Cancer 