The invention provides a method for identifying one or more genes in a cell of a cell type which interact with, e.g., modulate the effect of, an agent, e.g., a drug. For example, an identified gene may confer resistance or sensitivity to a drug, i.e., reduces or enhances the effect of the drug. The invention also provides STK6 and TPX2 as a gene that exhibits synthetic lethal interactions with KSP encoding a kinesin-like motor protein, and methods and compositions for treatment of diseases, e.g., cancers, by modulating the expression of STK6 or TPX2 gene and/or the activity of STK6 or TPX2 gene product. The invention also provides genes involved in cellular response to DNA damage, and their therapeutic uses.