The present invention discloses methods for detecting exposure of a living subject to genotoxic agents, testing sensitivity to a genotoxic agent, and determining DNA damage caused by exposure to an agent, comprising detecting the presence of FANCD2-containing foci from a sample collected from said subject. The presence of concentrated foci is indicative of DNA damage, and the degree of foci formation is correlated with degree of exposure. Diagnostic reagents contain a ligand that binds to human FANCD2 associated with a detectable label. Kits for detecting DNA damage in a biological sample contain such diagnostic reagents and signal detection components. The invention further discloses methods for identifying agents which modulate the ability of FANCD2-containing foci to form. Among other things, such agents are potentially useful chemosensitizing agents or may confer protection against damage caused by genotoxic agents.