Effectiveness of siRNA injected into brain in reducing the production of the enzyme tyrosine hydroxylase (TH) in mouse brain cells. TH expression in the cells is represented by the green fluorescence. A) Comparison of the intensity of TH fluorescence in the side of the brain treated with a control siRNA (left) and in the side treated with TH-siRNA (right). The intensity of the fluorescence in the control side of the brain is higher than in the TH-siRNA–treated side of the brain (see D). B) and C) Comparison of the number of TH-expressing cells in the same brain section shown in A. The number of TH-expressing cells is higher in the control side B than in the TH-siRNA–treated side C. D) The intensity of the fluorescence is greater in the control side than in the TH-siRNA– treated side.