Tumor cells with mutations in specific genes, including TLE1, attract neutrophils to the primary tumor site and increase the number of CTC-neutrophil clusters in the bloodstream. Expression of the cell adhesion molecule VCAM1 in the tumor cells mediates the formation of CTC-neutrophil clusters, presumably by binding to specific integrins present in neutrophils. These neutrophils express IL6 and IL1β, signals that promote CTC cell-cycle progression. Conversely, CTCs express neutrophil-stimulating factors such as G-CSF. CTC-neutrophil clusters are highly efficient in initiating metastasis and their presence correlates with poor outcome.