Variation in expression profiles between different isolation methods and visits can originate from both biological and technical sources. Inter-individual biological variation can arise from variation such as genetic variation, cellular composition, ethnicity, sex, genotype-environment interactions or physiological variation such as time of the day at which a sample was taken, diet and stress. The latter would also contribute to variability between multiple visits [6,7,9-11,28]. Technical variation can be caused by the different steps of the experiment such as sample preparation, isolation of cellular components, labelling, hybridisation and time to analysis [6,7,9].
