Yan, C. et al. Cell 7, 1903–1914.e14 (2019).

Researchers routinely transplant human cancer cells into immunocompromised mice to study tumor growth and metastasis. However, following the transplanted cells over time is not straightforward in mice, especially at single-cell resolution. Instead, Yan et al. use immunodeficient, transparent zebrafish to visualize xenografted cells. Prkdc–/–Il2rga–/– zebrafish lack both an adaptive immune system and NK cells, and the Casper background makes the fish transparent. The researchers transplanted cells from 16 different cancer cell lines, as well as xenografts from six human subjects, into these fish and were able to obtain tumor-bearing zebrafish in all cases. The transparent nature of the fish allowed them to monitor individual tumor cells over time and analyze their behavior. The zebrafish model also made it possible to study the effects of drugs alone and in combination on xenografted cells.