Using indirect immunolocalization and fluorescent in situ hybridization (FISH), Meng et al. showed that individual tumor cells in 12 primary breast cancers, and matched CTCs either expressed HER2, urokinase plasminogen activator receptor (uPAR), both, or neither [40], exemplifying CTC heterogeneity at the single cell level. This evidence concerns the gene ERBB2 and breast cancer.