LOS ANGELES — When cancers are treated, tumors may shrink but then come back. Now studies on three different types of tumors suggest a reason why: The cancers are fueled by stem cells that chemotherapy drugs don’t kill.

The findings, made by independent research teams that used mice to study tumors of the brain, intestines and skin, could change the approach to fighting cancers in humans, experts said.

Properties of these so-called cancer stem cells can be investigated so researchers can devise strategies for killing them off, said Luis F. Parada, a molecular geneticist at the University of Texas Southwestern Medical Center in Dallas and senior author of one of the studies published Wednesday.

Cancer researchers have long suspected, and some pioneering studies have strongly suggested, that specific cells within tumors are responsible for their continued growth.

The three papers published by the journals Nature and Science “really should seal the deal,” said cancer biologist Owen Witte, director of the Broad Stem Cell Research Center at the University of California, Los Angeles.

All three studies used molecular tricks that allowed scientists to mark certain tumor cells with bright colors. When these marked cells divided, all of the daughter cells were similarly colored, so researchers could see whether any old cell in a tumor can continue to fuel its growth or if only a subset of cells can. The three groups used different experimental approaches and kinds of cancer, but all of them found the latter to be true.

Parada’s group, whose work was published in Nature, studied an aggressive cancer called glioblastoma that arises when brain cells called glia turn rogue. The scientists started with a hunch: if a cancer stem cell existed, it would have biological similarities to the stem cells that normally exist in the brain.

To test whether this was true, the team created glioblastoma-prone mice whose brain stem cells glowed green. When those cells divided, their daughter cells contained some of the green dye too.

Sure enough, the mice developed brain tumors. When the researchers examined those tumors, they found a small number of green-glowing cells that weren’t actively dividing, unlike the rest of the tumor.

Next, the scientists treated the mice with a chemotherapy drug that kills rapidly dividing tumor cells. When the tumors grew back, as glioblastomas generally do, the scientists used other chemical tricks to see that the new cells were all descendants of the green-glowing cells that weren’t killed by the drug.

The next step was to see what would happen if the cancer stem cells were wiped out, a test that was possible because of the way the mice were genetically constructed.

Without the stem cells, the tumors never grew as large and the animals lived longer. The scientists concluded they had destroyed the wellspring of cells that renew the tumor when other cancer cells within the mass stop dividing.