can be any conventional atom or molecule used for diagnostic labeling examples are radioactive labels such as 125 i compounds technetium isotopes nmr reporter groups and fluorescent dyes the most preferred labels are hydrophobic labels such as fluorescent dyes most conventional fluorescent dyes happen to be hydrophobic which are incorporated into the cytoplasmic membrane of target cells as will be explained in more detail below labels can be attached to the hybrid protein according to conventional labeling techniques labels are used in an amount e g one or two label molecules per protein molecule which does not interfere with the binding or cell penetration functions of the hybrid protein molecule the labeled hybrid proteins of the invention can be used diagnostically to label a desired class of target cells a class determined by the specific polypeptide ligand which imparts the binding domain of the hybrid molecule the specific binding domain of the polypeptide ligand portion selectively binds to those cells the labeled molecule is then taken up by the cells via receptor mediated endocytosis and the label subsequently is delivered to the cell membrane and or the cytoplasm of the target cells the process by which a labeled hybrid protein of the invention can be incorporated into cells can be summarized as follows the labeled hybrid protein.