SPARC and cancer: (3) The most highly enriched CTC-specific transcripts shared by almost all classical CTCs encode extracellular matrix proteins, such as Sparc. (4) Aberrant expression in CTCs of this ECM gene product, which is normally abundant in the tumor stromal compartment, is observed in both mouse and human pancreatic CTCs, and its knockdown attenuates cancer cell migration and invasion in reconstituted systems.