http://www.pnas.org/content/110/2/791/F3.expansion.html
http://recherche.ircam.fr/equipes/repmus/RMPapers/ICMC99/
http://en.wikipedia.org/wiki/Sequence_motif
http://a-little-book-of-r-for-bioinformatics.readthedocs.org/en/latest/src/chapter3.html
http://www.springerimages.com/Images/RSS/1-10.1007_s10528-010-9406-4-2
http://telliott99.blogspot.co.uk/2009/06/motif-discovery-4.html