Publication of the International Union Against Cancer Publication de l’Union Internationale Contre le Cancer Int. J. Cancer: 71, 310 (1997) r 1997 Wiley-Liss, Inc. LETTER TO THE EDITOR Dear Sir, Identification of nude mice in tumorigenicity assays A test widely used to determine the effects of gene alterations on cell malignancy is implantation in vivo and monitoring of tumor formation. To perform such experiments with human cells, an immunocompromised animal host is required. Perhaps the most commonly used animal system for such tumorigenicity assays is the athymic ‘‘nude’’ mouse (Giovanella et al., 1974). Although this system is limited by false-negative results, it is widely accepted as an indicator of malignancy. Take rates can be improved by irradiation (30–60 Gy) or tumor cell implantation in Matrigel (Pretlow et al., 1991). One practical issue in dealing with these animals is precise identification of each individual mouse. Current practices on a small scale include the use of pen marks on tails, ear piercing or toe cutting. The shortcomings of these are loss of pen marks due to grooming and infringing on animal integrity by piercing or cutting. Here, I propose the use of a simple alternative method to label ‘‘nude’’ animals—namely, injection of colloidal carbone. Briefly, 5–10 µl of colloidal carbon suspension (China Ink, Pelikan, #211862) sterilized by autoclaving is injected s.c. with a 25G needle above the thigh of the animal (Fig. 1). Even smaller marks can be obtained by depositing a drop of ink on the skin and simply piercing through the drop with an empty needle. The ink easily penetrates the skin through capillary force. These marks are indelible, do not harm the animal and do not produce side effects such as bleeding or infections (over 500 mice tested so far). Usually, 4 or 5 animals are kept per cage, so a numbering system from 1 to 5 is suffıcient. This can be achieved with maximum of 2 injections per animal: one and two spots on the left hind leg (for #1 and #2), or on the right hind leg (for #3 and FIGURE 1 – Nude mouse #4 injected twice s.c. in the right hind leg with 5 µl of sterilized colloidal carbon suspension. #4), and one spot on each side for #5. Therefore, I recommend the use of this simple tattooing procedure for the marking of ‘‘nude’’ animals for small-scale tumorigenicity experiments. Yours sincerely, Erwin G. VAN MEIR Laboratory of Tumor Biology and Genetics, Neurosurgery Department, University Hospital (CHUV), 1011 Lausanne, Switzerland Received 5 November 1996 REFERENCES GIOVANELLA, B.C., STEHLIN, J.S. and WILLIAMS, L.J., Heterotransplantation of human malignant tumors in ‘‘nude’’ thymusless mice. II. Malignant tumors induced by injection of cell cultures derived from human solid tumors. J. nat. Cancer Inst., 52, 921–930 (1974). PRETLOW, T.G., DELMORO, C.M., DILLEY, G.G., SPADAFORA, C.G. and PRETLOW, T.P., Transplantation of human prostatic carcinoma into nude mice in Matrigel. Cancer Res., 51, 3814–3817 (1991).