Distribution of tumor necrosis factor (TNF)-α in a PC-3 xenograft tumor 4 days after implantation with GeneSeeds carrying plasmid DNA expressing TNF-α. This novel technology utilizes tiny metallic seeds containing a vector expressing TNF-α under the control of a constitutive promoter (cytomegalo viral promoter; CMV). Xenografts were grown to appropriate sizes and were implanted with vector-containing seeds. The diffusion rates of gene delivery in tumors were determined as a function of distance from the open ends of seeds by scoring positive cell numbers: the top to bottom panels represent tissue sections at 50 µm, 2,000 µm, 4,000 µm, and 6,000 µm in distance from the open end of seeds exhibiting 82%, 72%, 15%, and 5% of cells as TNF-α positive, respectively. The data show that the distributions of expressed gene product were highly localized around the ends of the seeds, gradually decreasing as a function of distance from the open ends of seeds. For details, see 655 et al. in this issue.