- Heterozygous ras mutations are preserved in serially passaged human tumor xenografts and established cell lines. Masuda, K., Kijima, H., Kim, H.M., Han, S.B., Ohnishi, Y., Sawa, N., Oshika, Y., Tokunaga, T., Tsuchida, T., Abe, Y., Hamana, T., Matsubayashi, H., Yamazaki, H., Tamaoki, N., Ueyama, Y., Nakamura, M. Oncol. Rep. (1999)