To cite package ‘Modstrings’ in publications use:

  Ernst F, Lafontaine D (2022). _Modstrings: implementation of
  Biostrings to work with nucleotide sequences containing modified
  nucleotides_. R package version 1.21.0.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {{Modstrings}: implementation of Biostrings to work with nucleotide sequences containing modified nucleotides},
    author = {Felix G.M. Ernst and Denis L.J. Lafontaine},
    year = {2022},
    note = {R package version 1.21.0},
  }