A differential expression screening method is provided for identifying a genetic element involved in a cellular process, which method comprises comparing: (a) gene expression in a first cell of interest; and (b) gene expression in a second cell of interest, which cell comprises altered levels, relative to physiological levels, of a biological molecule implicated in the cellular process, due to the introduction into the second cell of a heterologous nucleic acid directing expression of a polypeptide; and identifying a genetic element whose expression differs, wherein gene expression in said first and/or second cell of interest is compared under at least two different environmental conditions relevant to the cellular process.