I think this section would make more sense in the `NucleotideSequence` class docstring, as that's where you first reference IUPAC characters.

numpydoc references are described in section 10 of this page:

https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt

Can you please move this section and update the formatting? Note the numbering and strict indentation required by the `References` section.