In the [Adapterama paper][1], they are talking about a way to tag individuals within a library preparation for sequencing. 

At some point, they explain their primer and show this: 


`   iTru5_01_A: 5';AATGATACGGCGACCACCGAGATCTACAC     tag;ACCGACAA 3';ACACTCTTTCCCTA*C`

What is the star for in this sequence: 

`ACACTCTTTCCCTA*C`

This appears [in other protocols][2]. 


  [1]: http://www.biorxiv.org/content/biorxiv/early/2016/06/15/049114.full.pdf
  [2]: https://github.com/eacooper400/GenDivergence/blob/master/dRAD_LibraryPrep_Protocol.docx
It's some sort of modified nucleotide: phosphorothioate bond

See this page from [IDT][1]


  [1]: https://www.idtdna.com/pages/decoded/decoded-articles/core-concepts/decoded/2011/09/12/phosphorothioates