Abstract: The invention provides a method allowing to determine whether a subject diagnosed with cancer is sensitive or resistant to an anti-cancer treatment, based on the level of cells with RAD51 foci in a sample containing tumor cells isolated from said subject, wherein the subject has not received at 24 hours prior to the isolation of the sample, a chemotherapy selected from the group consisting of AC, FEC, ECF and navelbine/epirubicin, and wherein the sample has not been treated with a method that induces DNA damage before determining the level of cells with RAD51 foci.