Scientists have discovered new information about the origin of tumors. The scientists postulate that the initiation of a tumor and the type and aggressivity of the same depend on a specific combination of defects in several processes that safeguard cell integrity, such as DNA repair pathways and cell cycle check-points. The study also demonstrates that mice with a high degree of chromosomal instability and defective programmed cell death (apoptosis), two hallmarks of cancer, rarely develop tumors.