GRPR and gastrointestinal stromal tumor: Since overexpression of the GRPR has been reported in various cancer types, e.g. prostate cancer (Markwalder & Reubi, 1999), breast cancer (Halmos et al., 1995), gastrointestinal stromal tumors (GISTs) (Reubi et al., 2004) and other tumors (Reubi et al., 2002).