GPC1 and breast cancer: Similar to CA19‐9, GPC1 upregulation has also been observed in other cancer types.[17, 18, 19, 20, 21, 22, 23] Therefore, we measured the expression of GPC1 Exo‐mRNA and tMV‐mProtein in serum samples from late‐stage breast cancer (BC) patients (MSKCC, n = 31), plasma samples from late‐stage hepatocellular carcinoma (HCC) patients (National Health Research Institute (NHRI) Biobank, n = 11), and plasma samples from late‐stage esophageal cancer (EC) patients (Chang Guan Memorial Hospital (CGMH), n = 11).