The invention relates to a marking precursor incorporating a chelator or fluorination group for radiolabelling with 44Sc, 47Sc, 55Co, 62Cu, 64Cu, 67Cu, 66Ga, 67Ga, 68Ga, 89Zr, 86Y, 90Y, 90Nb, 99mTc, 111In, 135Sm, 140Pr, 159Gd, 149Tb, 160Tb, 161Tb, 165Er, 166Dy, 166Ho, 175Yb, 177Lu, 186Re, 188Re, 213Bi and 225Ac or with 18F, 131I or 211At, and one or two biological targeting vectors which are coupled to the chelator or fluorinating group via one or more squaric acid groups.