A member of the solute carrier protein family, sodium iodide symporter (NIS) is the only candidate involved in iodine transport and, therefore, its presence on the cell membrane matters for its function in relevant human tissues. In breast cancer, strong overexpression of NIS is seen, but it is predominantly localised in the cytosol, hindering the development of a NIS-directed targeted radioiodine therapy protocol. Membrane localisation of NIS is thought to be promoted by N-linked glycosylation. Now, Abhijit De and colleagues (Rathod et al., 2019) set out to understand the localisation pathway for NIS by using a breast cancer cell line that overexpresses NIS. Clonal derivatives from this line show cytosolic NIS localisation in the majority of cases, but several clones display NIS at the plasma membrane. The authors describe that the differences between these clonal cells are due to defective NIS trafficking through the secretory pathway. Correct intracellular trafficking and maturation of the core glycan is a requirement for glycoprotein localisation, and the authors find that inhibition of N-glycosylation disrupts the translocation of NIS from the secretory pathway to the plasma membrane. In addition, the authors identify that the activity of mannosidases is required for correct trafficking of NIS to the plasma membrane. This work establishes the molecular targets that are involved in defective NIS function in breast cancer cells and improve our understanding of NIS function as a potential therapeutic cancer target.