Changes in cell demographics can result in gene expression. Two scenarios which result in similar gene expression changes: (A) The cell type(s) within the sample are unchanged, but, over time, inactivated cells (colored blue) become activated and express a marker gene (colored red); (B) A second cell type already actively expressing the marker gene (red colored pentagons) migrates into the sample. The change in the marker gene expression is similar in both cases but results from a different reason.
Shoemaker et al. BMC Genomics 2012 13:460 doi:10.1186/1471-2164-13-460