CAR-T cell therapy is a new type of cancer therapy. It is based on equipping T cells with engineered receptors that enable the cells to recognize and kill cancer cells. Unfortunately, CAR-T cell therapy is still risky and to date has only been effective against a few types of cancer. Here, we use computational protein design to develop sensors that respond to small-molecule drugs. We insert these sensors into a modified CAR receptor to drug-dependently turn the CAR-T cells on or off. In this way, we aim to reduce side effects as the drugs allow precise dosing of CAR-T cell activity. This could also enable the treatment of other types of cancer where previous approaches have failed due to the high risk of severe side effects.