Cancer immunotherapy drugs, which spur the body’s own immune system to attack tumors, hold great promise but still fail many patients. New research may help explain why some cancers elude the new class of therapies, and offer some clues to a solution.

The study, published on Thursday in the journal Cell, focuses on colorectal and prostate cancer. These are among the cancers that seem largely impervious to a key mechanism of immunotherapy drugs.

The drugs block a signal that tumors send to stymie the immune system. That signal gets sent via a particular molecule that is found on the surface of some tumor cells.

The trouble is that the molecule, called PD-L1, does not appear on the surface of all tumors, and in those cases, the drugs have trouble interfering with the signal sent by the cancer.