Research conducted at Lawrence Livermore National Laboratory using human cells demonstrated similar changes in gene expression. That is the number of genes activated by high and low doses of radiation are different. It also demonstrated that there were a relatively small number of genes that responded to both high and low doses of radiation. Finally, they demonstrated that there were many more genes up-regulated by low doses of radiation than by high doses. The number of genes that were down-regulated were similar for the two doses used (2.0 Gy and 0.1 Gy) in these studies. Such studies also suggest that the mechanism of action for radiation action is different at high doses than is observed after low doses of radiation. Such differences would suggest that linear extrapolation of effects at the cell and molecular level are not possible. On the other hand, for single responding genes, the level of expression is linearly related to dose.