Our data identify Twist1 as a TNF induced repressor of the expression of Per1, Per2, Per3 and Dbp. Upon treatment with TNF and IL-1β NIH-3T3 fibroblasts and HT22 hippocampal neuronal cells show a striking upregulation of Twist1 and Twist2. These data are in agreement with recent reports showing TNF and IL-1β to induce both DNA binding proteins in different types of cells including fibroblasts, macrophages, breast epithelial cells, and various tumor cells [21, 26]. This evidence concerns the gene PER1 and neoplasm.