Exhibit 99.1 Lyell Immunopharma Reports Business Highlights and Financial Results for the Fourth Quarter and Full Year 2023 • On track to report initial clinical and translational data from lead CAR T-cell and TIL product candidates in 2024 • Received Orphan Drug Designation for LYL845 for the treatment of melanoma • Expect to submit IND for second generation ROR1-targeted CAR T-cell product in the first half of 2024 • Presented nonclinical data demonstrating the ability of Lyell’s novel genetic and epigene ...