Reference:
  Author: Ari Löytynoja and Michel C Milinkovitch
  Title: A hidden Markov model for progressive multiple alignment
  Journal: Bioinformatics
  Year: 2003
  Volume: 19
  Number: 12
  Pages: 1505-13
  DOI: 10.1093/bioinformatics/btg193
  PMID: 12912831
  URL: http://bioinformatics.oxfordjournals.org/content/19/12/1505
  eprint: http://bioinformatics.oxfordjournals.org/content/19/12/1505.full.pdf+html
