library_construction_protocol: CAR-T cells washed with PBS (Mg²+/Ca²+-free), centrifuged (300×g, 5 min), resuspended in PBS at 1×10^6 cells/mL for sequencing. R1 (read 1 primer sequence) are added to the molecules during GEM incubation. P5, P7, a sample index, and R2 (read 2 primer sequence) are added during library construction via End Repair, A-tailing, Adaptor Ligation, and PCR. The final libraries contain the P5 and P7 primers used in Illumina bridge amplification.