Le Calvez et al., 2006: "Subcutaneous microchip-associated tumours in B6C3F1 mice: A retrospective study to attempt to determine their histogenesis." Experimental and Toxicologic Pathology. 2006;57:255—265.
Summary: Microchips were implanted into 1,260 experimental mice for identification purposes as part of a larger study. Two years later, 4.1% of the mice had developed malignant tumors at the site of the microchip implantation. The cancers were directly attributed to the microchips.
Excerpts: "Most of the animals with microchip-associated tumors died prematurely...due to the size of the masses [or] the deaths were spontaneous and attributed to the masses." // "One of the most potentially serious disadvantages of the microchip implantation is the possibility that foreign-body-induced tumours may develop."