In a summary, CD81 is a novel partner interacting with CD44 in breast tumor-initiating cells and promotes exosome-induced self-renewal (mammosphere formation and signature markers), tumor cluster formation, and therefore enhancing tumor initiation and lung metastasis of TNBC with an unfavorable overall survival and metastasis-free survival (Figure 6—figure supplement 2). Here, CD81 is linked to breast neoplasm.