Cytokines like IL-6 and IL-8, which are secreted by osteosarcoma cells, play a role in recruitment and clonal expansion of CTCs in distant organs to promote osteosarcoma outgrowth and act as tumour-derived attractants of CTCs; this phenomenon is called ‘tumour self-seeding of CTCs’ and can accelerate tumour growth [126, 127]. The gene discussed is IL6; the disease is osteosarcoma.