CD44 is a cell-surface glycoprotein postulated to play a role in a variety of biological processes, including cell-to-cell and cell-to-matrix adhesion, lymphocyte homing and tumor cell metastasis. Several isoforms of CD44 have been identified in human cells, and the genesis of some of these isoforms has been attributed to alternative splicing. Understanding of mechanisms regulating CD44 alternative splicing may provide insights into diverse processes, including tumor-cell metastasis and lymphocyte homing. CD44 is widely expressed on many types of cells with mesodermal and hematopoietic origin, epithelial cells and a variety of tumors derived from these cells. Loss of CD44 expression has been linked to tumor invasion, metastasis and progression in carcinomas of breast, prostate, lung, ovary and malignant melanoma.