RET and thyroid tumor: Santoro et al. (1996) found that some RET/PTC1 transgenic mice developed thyroid tumors, but others developed only thyroid hyperplasia. Knostman et al. (2007) reported that doxycycline-induced expression of RET/PTC1 led to ERK phosphorylation in mice that carried a doxycycline-regulated RET/PTC1 transgene; however, thyroid lesions were not found in any of these mice.