The present invention relates to methods for detecting for the presence of an agent that putatively causes or potentiates DNA damage comprising subjecting a cell (containing a DNA sequence encoding a reporter protein operatively linked to a human GADD45α gene promoter and a human GADD45α gene regulatory element arranged to activate expression of the DNA sequence in response to DNA damage) to an agent; and monitoring the expression of the reporter protein from the cell. The invention also concerns expression cassettes, vectors and cells which may be used according to such a method and also modified media that may be employed in fluorescence assays and in preferred embodiments of the method of the invention.