RET and obesity due to melanocortin 4 receptor deficiency: RET (rearranged during transfection)-PTC (papillary thyroid carcinoma) is a common fusion gene in thyroid cancer that has been associated with various factors such as radiation exposure, genetic susceptibility, autoimmune diseases, obesity, and abnormal iodine intake.89, 90, 91, 92 The RET-PTC fusion gene is a fusion of the RET gene and the PTC gene to produce a new gene.