SLC5A5 and cancer: We also determined whether the observed signals in cancerous tissues were due to specific NIS uptake or were possibly generated by the enhanced permeability and retention effect (EPR), which is known to play a major role in tumour uptake of various agents delivered via the blood stream.54 Animals were therefore re-imaged 2 days after the first imaging session, but with prior administration of the competitive NIS substrate ClO4− before [99mTc]TcO4− injection to visualise cancer cells.