CD44 is the receptor for hyaluronic acid (HA). It mediates cell to cell and cell matrix interactions through its affinity for HA. Adhesion with HA plays an important role in cell migration, tumor growth and progression. Also involved in lymphocyte activation, recirculation and homing, and in hematopoiesis. High protein heterogeneity due to several alternative splicing and post translational modification events. CD44R represents a heterogeneous group of CD44 variant isoforms expressed constitutively in epithelial cells and monocyte lineage cells and can be upregulated in activated leukocytes. CD44R, and in particular CD44v6 containing isoforms, may be involved in leukocyte attachment to and rolling on endothelial cells, homing to peripheral lymphoid organs and to sites of inflammation. CD44v6 containing isoforms may be involved in tumor metastasis.