Thanks Nooshin and Sean I found this two links very elucidative for my question http://medbioinfo.uni- muenster.de/EuGESMACourse/DifferentialExpression.pdf http://medbioinfo.uni-muenster.de/EuGESMACourse/Exercises-2.R Cheers Marcelo -- Marcelo Mendes Brand?o Postdoc fellow Laborat?rio de Biologia Molecular de Plantas - ESALQ/USP Website: http://bioinfo.esalq.usp.br AtPIN: http://bioinfo.esalq.usp.br/atpin SKYPE: mmbrand Tel: (+55) 19 3429 4442