PDCD1 and triple-negative breast carcinoma: One study has shown that for human triple-negative breast cancer cells with mesothelin overexpression that can be targeted by CAR T cells, by counteracting the inhibitory effect of PD-1 on CAR-T cells through CRISPR-Cas9 ribonucleoprotein-mediated editing, PD-1hi population was significantly reduced with a minimal impact on the proliferation of T cells.