In high-grade B-cell lymphomas, nearly 50% of patients progress following CAR T treatment due to host-related factors affecting CAR T cell proliferation and persistence, as well as tumor-intrinsic factors, such as loss of CD19 epitope expression, trogocytosis, and other genomic alterations (e.g., CD19 mutations, chromothripsis, APOBEC mutational activity, and deletions of RHOA). The gene discussed is CD19; the disease is neoplasm.