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Yeah, this is why my comment led with “I trust the author”…

I’m surprised you need the full 4 bits to deal with ambiguous bases, but it probably makes sense at some lower level I don’t understand.

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This is because there's four bases and each can either be included or excluded from a given combination. So there are 4*2 = 16 combinations each of which with their own letter. In all honesty, these are pretty rarely used in practice these days except for N (any base) although they do sometimes show up when representing consensus sequences.

What do you mean that each base can be included or excluded? Isn't only one extra value needed? Sort of like nil?

Because there are notations for any combination of bases. There's a way to indicate "C or G", "A or T", "C, G, or A", etc.

Oh. So what's the grand total of all possible permutations of single and multiple (connected with an "or") values?

I'll also read through your links, thanks for posting them.

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