Overview of the RPM in vitro metastasis model. The RPM offers the advantage that 2D cell cultures in normal cell culture flasks can be used under conditions of simulated microgravity. The detachment of the cells and the subsequent microgravity-based spheroid formation (blue boxes) have many parallels to metastasis in the human body (green box), providing a good study model for the physiology of cancer spread and pharmaceutical tests. CD: cluster of differentiation, CTC: circulating tumor cells, GFR: growth factor receptor; ECM: extracellular matrix, EGFR: epithelial growth factor receptor; ESR1: estrogen-receptor 1, ICAM: Intercellular adhesion molecule, IL: interleukin, MMP: matrix metalloproteinase, VEGF: vascular endothelial growth factor. Parts of the figure were drawn using pictures from Biorender.com and Servier Medical Art.