Whole-genome bisulfite sequencing for chromosome 22 from Lister et al.
  This dataset represents chromosome 22 from the IMR90 cell line
  sequenced in Lister et al.  Only CpG methylation are included (there
  were very few non-CpG loci).  The two samples are two different
  extractions from the same cell line (ie. technical replicates), and
  are pooled in the analysis in the original paper.
  An object of class \code{BSseq}.
  All coordinates are in hg18.
  Obtained from \url{http://neomorph.salk.edu/human_methylome/data.html}
  specifically the files \url{mc_imr90_r1.tar.gz} and
  \url{mc_imr90_r2.tar.gz}.  A script which downloads these files and
  constructs the \code{BS.chr22} object may be found in
  \file{inst/scripts/get_BS.chr22.R}, see the example.
  R Lister et al.
  \emph{Human DNA methylomes at base resolution show widespread
    epigenomic differences}.
  Nature (2009) 462, 315-322.

script <- system.file("scripts", "get_BS.chr22.R", package = "bsseq")