The blocking studies of [18F]21 with FAPI-04 (Fig. 3e–f) showed a significant reduction in tumor uptake (14.13 ± 3.19%ID/g vs. 3.8 ± 0.89%ID/g), validating the FAP specificity of [18F]21 in vivo (Fig. 3e–f). The gene discussed is FAP; the disease is neoplasm.