Breast cancer is certainly a complex disease, driven by myriad genetic and lifestyle factors. But in the latest analysis of the DNA of breast tumors, researchers are heartened by the fact that the disease may be slightly simpler than they had thought. The Cancer Genome Atlas, a government project that is sequencing the genomes of dozens of cancers, found more than 30,000 mutations in 510 samples of breast tumors, but these fell into four major subtypes. One showed close ties to ovarian cancer, opening up the possibility that treatments for that cancer can also help breast-cancer patients, while another helps explain why some have better outcomes than others among women with HER-2 receptor tumors that are supposed to respond to drugs like Herceptin. The knowledge could translate into changes in the way doctors treat breast cancers and be the difference between surviving or becoming a victim of the disease.