To cite geneXtendeR in publications use:

  Khomtchouk BB, Koehler WC, Van Booven DJ, Wahlestedt C. Optimized
  functional annotation of ChIP-seq data. bioRxiv, 2018.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Optimized functional annotation of ChIP-seq data},
    author = {Bohdan Khomtchouk and William Koehler and Derek {Van Booven} and Claes Wahlestedt},
    journal = {bioRxiv},
    year = {2018},
    number = {R package version 1.8.0},
  }