To verify the impact of natural expression and overexpression of antigens on inducing CAR down-regulation in myeloid tumor cell lines, we conducted a co-culture assay using the U937 cell line, which naturally expresses the CD33 antigen and overexpresses it again after CD33 knockdown, with CD33 CAR T cells and found that both induced CAR down-regulation and the degree of down-regulation was basically the same. The gene discussed is CD33; the disease is myeloid neoplasm.