The present invention provides a kit of vectors comprising: (i) a first vector comprising a nucleic acid sequence encoding a first marker component； and (ii) a second vector comprising a nucleic acid sequence encoding a second marker component, wherein, when a cell is transduced with both the first and second vectors, the first and second marker components are expressed by the cell and associate forming a hetero- multimeric marker which is recognised by a cell sorting reagent whereas, when a cell is transduced with either the first or second vector alone, expression of the first or second marker component alone is not recognised by the cell sorting reagent.
