PLAUR and cancer: These circulating cancer cells likely originate, at least partially, from the uPAR positive “budding” cancer cells sporadically found within the tumor stromal microenvironment such as, for example, in colon adenocarcinomas or gastric cancers, in contrast to the bulk of the primary uPAR-negative tumor [130,131], as shown by the IHC images in Figure 5a,b.