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Figure 4 Ex vivo immunohistochemical staining of transgene products. Shown are microscopic images of a representative sample from a myocardial area injected with adenovirus carrying HSV1-sr39tk reporter gene together with the VEGF₁₂₁ gene (Ad_Tk-VEGF). Immunostaining for HSV1-tk (A and C) and VEGF (B and D) was performed in adjacent slices. Dark brown color indicates presence of the respective gene product. Low magnification (A and B) shows corresponding expression of both transgene products in the same tissue region. High magnification (C and D) identifies HSV1-tk reporter gene product in cytosol and nuclei, whereas VEGF is found in cytosol only, partly in a vesicle-like pattern. Subtle differences in strength and cellular localization of the immunostaining signal are explained by gene product characteristics (HSV1-TK is an intracellular enzyme, VEGF is a secretable substance).