Several miRNAs, lncRNAs, mtDNA, and proteins—including VEGF, nitric oxide synthase 2 (NOS2), collagen type III alpha 1 (COL3A1), slit homolog 3 (SLIT3), follistatin (FST), neuronatin (NNAT), and protocadherin 17 (PCDH17)—serve as key active substances in the paracrine signaling of circulating tumor cells (cTCs), as shown in Figure 2 and Table 2. Here, PCDH17 is linked to neoplasm.