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CD44 is a transmembrane glycoprotein that plays crucial roles in various cellular processes. It is a type I transmembrane protein and a member of the cartilage link protein family. The human CD44 gene contains 19 exons and is located on chromosome 11p13. Exons 1–5, 16, 17, and 19 are constant and common to all CD44 isoforms, encoding the standard form of CD44 （CD44s）. Variant forms （CD44v） arise from the combination of constant exons with variable exons 7–15 through alternative mRNA splicing. CD44 proteins have a common structure consisting of four major parts: the extracellular hyaluronan-binding domain （HABD）, the stem region, the transmembrane segment, and the cytoplasmic region. The HABD confers binding capability to CD44 mainly for hyaluronan （HA）, but also for collagen, laminin, and fibronectin. CD44 is involved in cell–cell and cell–matrix adhesion, cell proliferation, migration, hematopoiesis, and lymphocyte activation. It is overexpressed in several cell types, including cancer stem cells, and its variant isoforms are thought to play a role in cancer development and progression.