A selection method for selecting from a population of cells one or more selectable genetically transformed cells is described. The population of cells comprises selectable genetically transformed cells and possible non-transformed cells. Each of the selectable genetically transformed cells comprises a first expressable nucleotide sequence encoding a first expression product; and optionally a second expressable nucleotide sequence encoding a second expression product and/or a third expressable nucleotide sequence encoding a third expression product. A component is utilisable by the selectable genetically transformed cells by action of the first expressable nucleotide sequence or the first expression product and optionally by action of the optional second expressable nucleotide sequence or the optional second expression product and/or by action of the optional third expressable nucleotide sequence or the optional third expression product. The component can be present in an amount that is toxic to the non-transformed cells.
