Introduction to this special issue "Breast Cancer Metastasis".

Breast cancer remains the most common malignancy and most frequent cause of cancerrelated death in women, a devasting reality that annually claims more than 600,000 lives across the globe[1]. The vast majority of deaths due to breast cancer are attributed to metastasis and its associated relapse[2,3], which typically transpires in patients ~5-20 years after their initial diagnosis[4]. Although metastasis is the most lethal characteristic of breast cancer, our understanding of the molecular mechanisms that govern this event remains incomplete, a stark reality reinforced by the finding that diagnosis of distant-stage disease has remained unchanged over the course of the last two decades[5].