Abstract: There is disclosed an assay to determine the effect of an applied agent or influence or other stimulus on the sub-cellular localisation of the expressed product of a nucleotide sequence (“the nucleotide sequence under investigation”), comprising: (i) transfecting a cell population with a cloned expression vector comprising a composite nucleotide sequence encoding the nucleotide sequence under investigation and a detectable tag, arranged such that the product of the expressed composite nucleotide sequence includes the expressed detectable tag; (ii) applying the applied agent, influence or other stimulus to at least a portion of the transfected cell population; (iii) observing the sub-cellular location of the detectable tag in the transfected cell population and comparing the observed sub-cellular localisation to the sub-cellular localisation of the detectable tag in the cell population in the absence of said applied agent or influence.