The first proof-of-principle studies with [99mTc]Tc-RP527 ([99mTc]Tc-N3S-Gly-5aVa-BBN(7-14), 5aVa: 5-aminovaleric acid; Figure 6) in a small number of metastasized prostate and breast cancer patients applying SPECT established the feasibility of GRPR-targeted diagnostic imaging of these tumors and their metastases. The gene discussed is GRPR; the disease is breast cancer.