. "Serum-induced levels in fibroblasts show circadian oscillations. Maximum levels after 1 hour stimulation, minimum levels after 12 hours. Another peak is then observed after 20 hours. Protein levels show maximum levels at 6 hours, decrease to reach minimum levels at 20 hours, and increase again to reach a second peak after 26 hours. Levels then decrease slightly and then increase to maximum levels at 32 hours. Levels of phosphorylated form increase between 3 hours and 12 hours." . . .