NEW YORK — Scientists reported this week that they have completed a major analysis of the genetics of breast cancer, finding four major classes of the disease. They hope their work will lead to more effective treatments, perhaps with some drugs already in use.

The new finding offers hints that one type of breast cancer might be vulnerable to drugs that already work against ovarian cancer. The study, published online Sunday by the journal Nature, is the latest example of research into the biological details of tumors, rather than focusing primarily on where cancer arises in the body. The hope is that such research can reveal cancer’s genetic weaknesses for better drug targeting.