Tissue-level phase transitions are emerging as a crucial mechanism in tumour development and metastasis. When becoming invasive, epithelial tumours undergo a transition from a solid-like state to a more fluid-like one. Although the contributions of cell adhesions, traction forces and cell migration for such behaviour are known, the exact biophysical and molecular mechanisms controlling these transitions are not fully understood. Here we show that breast cancer cell fluidity is regulated by IRSp5