Both [99mTc]TEC and [18F]FDG were able to follow tumor progression in the PyMT model from early stages on, even though at later stages, [99mTc]TEC uptake as well as NIS staining tended to decrease in advanced breast cancer lesions, in contrast to [18F]FDG measurements of MTV that increased until the end of the period of observation. This evidence concerns the gene SLC5A5 and breast cancer.