The compound (W-Z-M) according to the invention comprises a component (M) chosen from the group consisting of markers and therapeutic agents possessing an intracellular active site (A.S.), linked to a ligand (W-Z) comprising an arm (Z) linked to a terminal group (W), characterized in that the linkage between the arm (Z) of the ligand (W-Z) and the component (M) prevents intracellular entry of the compound (W-Z-M) and/or inhibits expression of the marker (M), in that said linkage can be selectively cleaved by factors secreted by target cells so as to permit expression of the marker (M) or entry of the therapeutic agent (M) into said target cells, and in that the terminal group (W) provides for the stability of the compound (W-Z-M) in the serum and circulating blood.