Fibroblast activation protein (FAP) isa leading theranostic biomarkerbecause of its crucial role in angiogenesis, growth, and progressionin cancer.1 FAP is a transmembrane serineprotease overexpressed by cancer-associated fibroblasts (CAFs) inover 90% of epithelial cancers,2 includingbreast,3,4 colon,4,5 and pancreatic4,6 tumors. The gene discussed is FAP; the disease is cancer.