Human cancers can engraft into zebrafish embryos and larvae. Hematoxylin and Eosin stained cross section of a juvenile zebrafish that had been transplanted with MDA-435 adenocarcinoma cells. Cells were injected intraperitoneally and imaged at 5 days post-transplantation (Stoletov et al., 2007). Adenocarcinoma cells invaded the body wall (arrow, A). Transplanted Metastatic WM-266-4 melanoma cells formed pigmented masses in the intestinal wall by 7 days post-injection (Haldi et al., 2006). Lateral view (B) and ventral view (C) of the same fish. RFP-labeled human U251 glioblastoma cells can engraft into larval fish (D-I). Two U251-RFP glioblastoma cells transplanted into a 2 day old zebrafish embryo proliferate over time (Geiger et al., 2008). 2 days (D,E), 4 days (F,G), and 9 days post-transplantation (H,I).