Hi Tim, There are 4^4 = 16 possibilities of forming a 4-mer, like ATTG in your example. So what is the probability that you pick any position k in your genome of length L and find that to be ATTG? It is 1/16. Now, what is the probability of finding it at position 1, or 2, or ... k, or ..., L? If you excuse the boundary conditions, and this is perfectly fine for short motifs and long genomes, it would be (1/16 + 1/16 + ...) L times, or L/16. I agree, this is an approximation, but works pretty well actually. Regards, - Al. [[alternative HTML version deleted]]

:-D I caught that one two seconds after hitting "send" ... of course I meant 4^4 = 256. But you get the idea. - Al. [[alternative HTML version deleted]]

:-D I caught that one two seconds after hitting "send" ... of course I meant 4^4 = 256. But you get the idea. - Al. [[alternative HTML version deleted]]