The present invention provides assays for identifying drug candidates that regulate cellular senescence. In particular, the present invention relates to the use of ATRX foci as a biomarker for determining whether one or more drug candidates regulate senescence in a cell line. In one non-limiting embodiment, the present invention provides assays to identify compounds that can be used in a combinatorial cancer treatment. The present invention is based, at least in part, on the discovery that the number of ATRX foci increases in cells that undergo senescence. Accordingly, in non-limiting embodiments, the present invention provides for assays and kits for identifying drug combinations that may be useful in treating subject that have cancer and for identifying compounds that may be useful in treating age-related diseases.