A study by Kim et al. in breast carcinoma and melanoma models showed that cytokines such as interleukin-6 (IL-6) and interleukin-8 (IL-8) might be tumor-derived attractants of CTCs, which could induce the clonal expansion of CTCs in distant organs as well as attract the CTCs back to a primary tumor; this phenomenon is termed as “tumor self-seeding by CTCs” [45, 46]. Here, IL6 is linked to breast carcinoma.