Provided herein are methods, kits, and compositions for stratifying and treating subjects, e.g., subjects having cancer. In some examples, the methods involve use of a radiolabelled heavy chain variable domain derived from a heavy chain antibody (VHH), or a functional fragment thereof, as both a screening agent and a treatment agent. In some examples, the VHH, or a functional fragment thereof, that is radiolabelled with a radioisotope that is both a γ-emitter and β-emitter.