The CD44 antigen is a cell-surface glycoprotein involved in cell–cell interactions, cell adhesion and migration. CD44 is expressed in a large number of mammalian cell types. It participates in a wide variety of cellular functions including lymphocyte activation, recirculation and homing, hematopoiesis, and tumor metastasis. CD44, along with CD25, is used to track early T cell development in the thymus. CD44 expression is an indicative marker for effector-memory T-cells. Memory cell proliferation （activation） can also be assayed in vitro with CFSE chemical tagging. In addition, variations in CD44 are reported as cell surface markers for some breast and prostate cancer stem cells. In breast cancer research CD44+/CD24- expression is commonly used as a marker for breast CSCs and is used to sort breast cancer cells into a population enriched in cells with stem-like characteristics and has been seen as an indicator of increased survival time in epithelial ovarian cancer patients.