Renowned Massachusetts Institute of Technology cancer biologist Robert Weinberg is staking part of his considerable reputation on a bold theory that has divided the cancer field. Weinberg and others contend that tumors contain a small number of cells that are distinctive because they resemble the stem cells that give rise to normal tissues. These cancer seeds, able to resist chemotherapy and spring back months or years after treatment, may explain the tragic relapses people often experience. Give patients a drug that targets these cancer stem cells, the thinking goes, and the disease can be kept under control. Verastem Inc., a company Weinberg co-founded, is one of several that are launching a new round of clinical trials to find out whether the theory actually works. Beyond the promise of changing cancer care, the financial stakes are huge. But as many in the field acknowledge, it may be difficult to draw definitive conclusions from these trials