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. The standard isoform, designated CD44s, comprising exons 1–5 and 16–20 is expressed in most cell types. CD44 splice variants containing variable exons are designated CD44v. Some epithelial cells also express a larger isoform (CD44E), which includes exons v8–10. CD44 is a receptor for hyaluronic acid and internalizes metals bound to hyaluronic acid and can also interact with other ligands, such as osteopontin, collagens, and matrix metalloproteinases (MMPs). CD44, along with CD25, is used to track early T cell development in the thymus. D44 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.