First, ProCA1.GRPR enabled unprecedented sensitivity for the non-invasive detection of GRPR at low expression levels (2 × 104 receptors/cell) in H441 tumors and middling expression (4 × 105 receptors/cell) in PC3 prostate tumor using MRI for tumor xenografted mice with 8-fold lower injection dosage than that of Gd-DTPA. Here, GRPR is linked to neoplasm.