http://www.ncbi.nlm.nih.gov/pmc/articles/PMC310692/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC145848/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3279888/
http://www.ncbi.nlm.nih.gov/pubmed/9196743
http://www.ncbi.nlm.nih.gov/pubmed/15500921