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.

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