CD19 and leukemia: CTX120 in multiple myeloma; anti‐mesothelin CAR‐T cells modified by CRISPR/Cas9 in solid tumors; CTX110 in B‐cell malignancies; UCART019 in CD19+ leukemia and lymphoma; PD‐1 knockout T cells modified by CRISPR/Cas9 in esophageal cancer