A method for detecting the binding between a first polypeptide (A) and a second polypeptide (B) in a cell comprising: i) providing a cell that expresses: - (a) a polypeptide (GFP1-9) comprising an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:1-4 - (b) a first fusion protein wherein the polypeptide (A) is fused to a polypeptide (GFP10) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:5-7 - (c) a second fusion protein wherein the polypeptide (B) is fused to a polypeptide (GFP11) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:8-9 - and (d) an intrabody which binds to the reconstituted GFP when the complex is formed by the self-assembly of the first, second and third polypeptides (a), (b) and (c), Wherein the intrabody (d) is not able to bind to the GFP1-9, GFP10 and GFP11 polypeptides by themselves ii) detecting the fluorescence wherein when the fluorescence is detected it is concluded that the polypeptide (A) binds to polypeptide (B) and where the fluorescence is not detected it is concluded that the polypeptides (A) does not bind to polypeptide (B). i) providing a cell that expresses: - (a) a polypeptide (GFP1-9) comprising an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:1-4 - (b) a first fusion protein wherein the polypeptide (A) is fused to a polypeptide (GFP10) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:5-7 - (c) a second fusion protein wherein the polypeptide (B) is fused to a polypeptide (GFP11) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:8-9 - and (d) an intrabody which binds to the reconstituted GFP when the complex is formed by the self-assembly of the first, second and third polypeptides (a), (b) and (c), Wherein the intrabody (d) is not able to bind to the GFP1-9, GFP10 and GFP11 polypeptides by themselves - (a) a polypeptide (GFP1-9) comprising an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:1-4 - (b) a first fusion protein wherein the polypeptide (A) is fused to a polypeptide (GFP10) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:5-7 - (c) a second fusion protein wherein the polypeptide (B) is fused to a polypeptide (GFP11) having an amino an amino acid sequence having at least 90% of identity with the amino acid sequence selected from the group consisting of SEQ ID NO:8-9 - and (d) an intrabody which binds to the reconstituted GFP when the complex is formed by the self-assembly of the first, second and third polypeptides (a), (b) and (c), Wherein the intrabody (d) is not able to bind to the GFP1-9, GFP10 and GFP11 polypeptides by themselves ii) detecting the fluorescence wherein when the fluorescence is detected it is concluded that the polypeptide (A) binds to polypeptide (B) and where the fluorescence is not detected it is concluded that the polypeptides (A) does not bind to polypeptide (B). <EOS>