I want to calculate the possibility, that this occures by chance to make a declaration about the clusters exceptionality.

Okay. I guess you know your business, but ... :)

An (over)estimate of the number of human cells that have ever been is 100 billion humans * 100 trillion cells each = 10^25. So, it seems to me that a probability of occurrence much below 1/10^25--ie. a one-time mutation amongst the entire human biomass--is as close to zero probability as makes no difference.

And anything 10^75 less frequent than that, just a meaningless statistic. Even if combined with other meaningful probabilities; if additive it will change the result by such a tiny amount as to be entirely swamped by the noise of experimental error; if multiplicative, it will reduce certainty to impossibility.

Of course, humans aren't the only species. Drosophila are billions of times more populous, but then their genomes is 200,000 times smaller. You're still talking occurrence due to neutrino damage rather evolution.

I'll shut up now :)