Blanchard et al., 1999: "Transponder-induced sarcoma in the heterozygous p53+/- mouse." Toxicologic Pathology. 1999;27(5):519--527.
Summary: 177 genetically modified mice were implanted with microchips for identification purposes as part of a chemical compound study. After six months, 18 of the mice (10.2%) had developed malignant tumors ("undifferentiated sarcomas") around the microchips. The tumors occurred in both experimental and control animals.
Excerpts: "There was an unequivocal association between the [microchip implant] transponder and sarcoma that was unrelated to drug treatment." // "The presence of the foreign body [microchip transponder] may elicit tissue reactions capable of generating genotoxic byproducts."