Engineered T cell therapies are revolutionizing cancer treatment by achieving long-lasting remission in blood-related cancers, such as leukemia and lymphoma. These therapies involve removal of patient T cells, ?reprogramming? them to attack cancer cells, and then transferring them back into the patient. Targeted gene inactivation (knockout) using CRISPR-Cas9 can enhance T cell activity (1, 2) and has the potential to expand cell therapy applications. Until now, it has been unknown whether CRISPR-Cas9?edited T cells would be tolerated and thrive once reinfused into a human. On page 1001 of this issue, Stadtmauer et al. (3) present data from a phase 1 clinical trial (designed to test safety and feasibility) on the first cancer patients treated with CRISPR-Cas9?modified T cells. The findings represent an important advance in the therapeutic application of gene editing and highlight the potential to accelerate development of cell-based therapies.