B2M and graft versus host disease: CRISPR/Cas9 technology also can be used to enhance anti-tumor effect of CAR-T cells by optimizing the manufacture of CAR-T cells and produce allogeneic CAR-T cells without graft-versus-host disease (GVHD) by disrupting T-cell receptor (TCR) beta chain and beta-2-microglobulin (B2M, an essential subunit of the HLA-I) (16, 17).