In recent years geneticists have started looking at how genetic differences between individuals affect gene expression. Different levels of expression generally correlate with variations in regulatory genes.
Emmanouil Dermitzakis and Stylianos Antonarakis, now both at the University of Geneva Medical School in Switzerland, and their team broke this down further by looking at how gene expression in different cell types derived from 75 people correlated with variations in their genomes. They took umbilical cord blood from pregnant women and cultured three types of cell for each. Comparing expression between individuals for each cell type, they found that 69–80% of gene variants affect expression levels in a manner specific to the cell type, suggesting that looking at just one tissue type is insufficient when comparing individuals.