['Relaxin (RLX or RLN) levels are increased in cases of human breast cancer and has been shown to promote cancer cell migration in carcinoma cells of the breast; however, the cellular mechanisms of relaxin exposure in breast cancer cells are not fully understood. In human breast cancer cells, relaxin was shown to downregulate the metastasis-promoting protein S100A4, a highly significant prognostic factor for poor survival in breast cancer patients. RLX was also found to enhance in-vitro invasiveness of breast cancer cell lines by induction of matrix metalloproteinases (MMPs) expression. The aim of this study was to investigate the effects of relaxin on breast cancer cell invasion by S100A4 dependent MMPs pathway.', 'The human breast cancer MDA-MB-231 cells were treated with 100-500 µg/L porcine RLX, or/and transfected with S100A4 siRNA (20 ng), or/and treated with MMPs inhibitor FN439 (0.3 nM).', 'We observed that incubation with porcine RLX increases in-vitro cell invasion and in vitro invasiveness. Enhanced invasiveness was accompanied by up-regulation of S100A4 and MMP-2 and MMP-9. The relaxin-induced increase in cell invasion was blocked almost when S100A4 expression was diminished using an S100A4 small interfering RNA knockdown approach or when MMPs was inhibited by MMPs inhibitor FN439. The relaxin-induced increase in MMP-2 and MMP-9 expression was blocked when S100A4 was inhibited by S100A4 siRNA transfection.'] Does relaxin enhance in-vitro invasiveness of breast cancer cell lines by upregulation of S100A4/MMPs signaling?