I have to calculate possibilities like 1/10^100.

Hm. Odds that small are so close to zero as to be meaningless.

There are estimated to be 10^80 atoms in the universe. If one of them had gone missing (or an extra one had materialised into existance) at some point after matter crystallised out of the big bang, the affect the absence or presence of that one atom would have had on the form of the universe would be undetectable at any scale.

And the affect that a 1/10^100 change in probability will have (on anything) is 100 million trillion times less significant.

To try and put that into perspective. If a casino dice had 1/10^100 bias, it would undetectable by any mechanism. I would guess that every single dice ever produced has been, is, and always will be biased billions of times more significantly. A single missing electron from the face of the dice will bias it far more significantly than that.

Indeed, the bias of every dice ever made probably varies by bllions of times more than that, second by second, due to moisture absorption, evaporation or radioactive decay.

If every human being that ever existed, threw every dice that ever existed, once every microsecond, for their entire lives and you managed to record every single throw, you would not be able to detect if such a rare event had occurred or not.

So what is the point of knowing such probabilities? Even knowing that they are not zero is useless information.

Of course, my problem is not the die itself. My goal is to asses gene clusters.
Imagine 10000 different genes. Each gene can have the values from 1, 10 or 100 (lets say it depends on their size). So I have:
500 genes with value 1
2000 genes with value 10
7500 genes with value 100
If I look at a certain gene cluster comprising 100 genes with an average value of 2.8 I want to calculate the possibility, that this occures by chance to make a declaration about the clusters exceptionality.
And in this context, possibilities are often <1/10^100.
I compare different clusters compairing the log10 of the reciprocal possibility.

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 :)

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.