The present invention relates to a method for the detection of the presence or absence of a target in a cell, the method comprising the steps of: (a) introducing into a cell (i) a first molecule comprising or consisting of a first single-domain antibody binding to a first portion of the target to be detected and a first detection moiety or a nucleic acid encoding said first molecule in expressible form and (ii) a second molecule comprising or consisting of a second single-domain antibody binding to a second portion of said target to be detected and a second detection moiety or a nucleic acid encoding said second molecule in expressible form, wherein said first portion and said second portion are spatially distinct to allow simultaneous binding of said first and said second single-domain antibody to said target and wherein said first and said second detection moiety are in combination suitable for generating a detectable fluorescent signal occurring as a consequence of fluorescent moiety Förster resonance energy transfer (FRET) or bimolecular fluorescence complementation (BiFC) when in spatial proximity of each other upon binding of said first molecule to said first portion and said second molecule to said second portion of the target to be detected (b) subjecting the cell of step (a) to conditions suitable for generating said detectable fluorescent signal and (c) analysing the cell of step (b) for the presence of said detectable fluorescent signal generated by way of fluorescent moiety Förster resonance energy transfer (FRET) or bimolecular fluorescence complementation (BiFC) occurring between said first and said second detection moiety, and/or for a decrease in fluorescence lifetime of the detection moiety acting as energy donor as a consequence of Förster resonance energy transfer (FRET) occurring between said first and said second moiety in comparison to a control, wherein if said detectable fluorescent signal and/or said decrease in fluorescence lifetime can be
