For anyone who wants to see the answer without looking at the, uhh, medical images, here's a quote: "Pluripotent stem cells including human induced pluripotent

For anyone who wants to see the answer without looking at the, uhh, medical images, here's a quote: "Pluripotent stem cells including human induced pluripotent stem cells have a unique property of being able to generate teratomas when injected in rodents in the research laboratory.[62] For this reason, the so-called "teratoma assay" is one of the gold-standard validation assays for pluripotent stem cells.[63] Because differentiated human pluripotent stem cells are being developed as the basis for numerous regenerative medicine therapies, there is concern that residual undifferentiated stem cells could lead to teratoma formation in injected patients, and researchers are working to develop methods to address this concern.[64]"