The Grand Locus / Life for statistical sciences

the Blog

## Longest runs and DNA alignments

Update: The key assumption of the approximation below is that $nq$ is a relatively big number. This happens if the read is very long ($n$ is large) and/or the eror rate is high ($q$ is large). While these assumptions are reasonable with PacBio or Oxford Nanopore technologies, they are not for Illumina reads. In this case, I recommend using the formula based on stick-breaking from this post (it also holds for PacBio and Oxford Nanopore by the way).

The problem of sequence alignment gets a lot of attention from bioinformaticians (the list of alignment software counts more than 200 entries). Yet, the statistical aspect of the problem is often neglected. In the post Once upon a BLAST, David Lipman explained that the breakthrough of BLAST was not a new algorithm, but the careful calibration of a heuristic by a sound statistical framework.

Inspired by this idea, I wanted to work out the probability of identifying best hits in the problem of long read alignments. Since this is a fairly general result and that it may be useful for many similar applications, I post it here for reference.

### Longest runs of 1s

I start with generalities on...

## Once upon a BLAST

The story of this post begins a few weeks ago when I received a surprising email. I have never read a scientific article giving a credible account of a research process. Only the successful hypotheses and the successful experiments are mentioned in the text — a small minority — and the painful intellectual labor behind discoveries is omitted altogether. Time is precious, and who wants to read endless failure stories? Point well taken. But this unspoken academic pact has sealed what I call the curse of research. In simple words, the curse is that by putting all the emphasis on the results, researchers become blind to the research process because they never discuss it. How to carry out good research? How to discover things? These are the questions that nobody raises (well, almost nobody).

Where did I leave off? Oh, yes... in my mailbox lies an email from David Lipman. For those who don’t know him, David Lipman is the director of the NCBI (the bio-informatics spearhead of the NIH), of which PubMed and GenBank are the most famous children. Incidentally, David is also the creator of BLAST. After a brief exchange on the topic of my previous...