A method of detecting the presence, amount or subcellular location of an antigenic structure of interest in a living cell, comprising the steps of: (a) (i) expressing a fusion protein directed to the antigenic structure of interest in said cell or (ii) introducing a fusion protein directed to the antigenic structure of interest and coupled to a (poly)peptide capable of transducing into said cell; wherein said fusion protein comprises a first (poly)peptide sequence comprising the variable region of a heavy chain antibody of Camelidae and a second (poly)peptide sequence, which is a detectable (poly)peptide, preferably from a fluorescent or chromophoric protein, wherein said 1. first (poly)peptide sequence is encoded by the nucleic acid sequence of SEQ ID NO: 2 or encoded by a nucleic acid sequence with at least 70% sequence identity thereto; and 2. second (poly)peptide sequence is a detectable protein; preferably a. the green fluorescent protein from Aequorea victoria encoded by the nucleic acid sequence of SEQ ID NO: 7, or a fluorescent mutant or fragment thereof; b. the red fluorescent protein from Discosoma (DsRed) encoded by the nucleic acid sequence of SEQ ID NO: 9, or a fluorescent mutant or fragment thereof; or c. a functional homologue of (a) or (b) with at least 80% sequence identity which retains the fluorescent activity of the fluorescent protein; (b) revealing the presence, amount or subcellular location of said antigenic structure of interest, if any, in said cell by means of said detectable protein; wherein said first (poly)peptide sequence is located N-terminally of said second (poly)peptide sequence, said sequences being optionally separated by a linker of at least one amino acid residue, wherein said method is not a diagnostic method practiced on the human or animal body (a) (i) expressing a fusion protein directed to the antigenic structure of interest in said cell or (ii) introducing a fusion protein directed to the antigenic structure of interest and coupled to a (poly)peptide capable of transducing into said cell; wherein said fusion protein comprises a first (poly)peptide sequence comprising the variable region of a heavy chain antibody of Camelidae and a second (poly)peptide sequence, which is a detectable (poly)peptide, preferably from a fluorescent or chromophoric protein, wherein said 1. first (poly)peptide sequence is encoded by the nucleic acid sequence of SEQ ID NO: 2 or encoded by a nucleic acid sequence with at least 70% sequence identity thereto; and 2. second (poly)peptide sequence is a detectable protein; preferably a. the green fluorescent protein from Aequorea victoria encoded by the nucleic acid sequence of SEQ ID NO: 7, or a fluorescent mutant or fragment thereof; b. the red fluorescent protein from Discosoma (DsRed) encoded by the nucleic acid sequence of SEQ ID NO: 9, or a fluorescent mutant or fragment thereof; or c. a functional homologue of (a) or (b) with at least 80% sequence identity which retains the fluorescent activity of the fluorescent protein; (i) expressing a fusion protein directed to the antigenic structure of interest in said cell or (ii) introducing a fusion protein directed to the antigenic structure of interest and coupled to a (poly)peptide capable of transducing into said cell; 1. first (poly)peptide sequence is encoded by the nucleic acid sequence of SEQ ID NO: 2 or encoded by a nucleic acid sequence with at least 70% sequence identity thereto; and 2. second (poly)peptide sequence is a detectable protein; preferably a. the green fluorescent protein from Aequorea victoria encoded by the nucleic acid sequence of SEQ ID NO: 7, or a fluorescent mutant or fragment thereof; b. the red fluorescent protein from Discosoma (DsRed) encoded by the nucleic acid sequence of SEQ ID NO: 9, or a fluorescent mutant or fragment thereof; or c. a functional homologue of (a) or (b) with at least 80% sequence identity which retains the fluorescent activity of the fluorescent protein; a. the green fluorescent protein from Aequorea victoria encoded by the nucleic acid sequence of SEQ ID NO: 7, or a fluorescent mutant or fragment thereof; b. the red fluorescent protein from Discosoma (DsRed) encoded by the nucleic acid sequence of SEQ ID NO: 9, or a fluorescent mutant or fragment thereof; or c. a functional homologue of (a) or (b) with at least 80% sequence identity which retains the fluorescent activity of the fluorescent protein; (b) revealing the presence, amount or subcellular location of said antigenic structure of interest, if any, in said cell by means of said detectable protein;