- Ulcer formation and associated tumor production in multiple sites within the stomach and duodenum of rats treated with N-methyl-N'-nitro-N-nitrosoguanidine. Takahashi, M., Shirai, T., Fukushima, S., Ito, N., Kokubo, T., Furukawa, F., Kurata, Y. J. Natl. Cancer Inst. (1981)