Provided is a method for screening for effective agents for the treatment of cancer which method comprises selecting a putative agent that is likely to disrupt a function mediated by a critical normal gene product, which function is required for the successful division and continued cell survival of cancer cells, and which function is not required for the successful division and continued cell survival of control cells, treating a cancer cell sample and a control cell sample with a purative agent and determining the cytotoxic effect of, and/or the growth inhibitory effect of the putative agent on these samples, and identifying an effective agent as an agent which is more cytotoxic to, and/or more inhibiting to the growth of the cancer cell sample than the control cell sample. This invention further provides an effective agent identified by this screening method and a method of treating a cancer patient with such an agent.