**2.3 CD44**

CD44 is a surface glycoprotein and a receptor for hyaluronic acid, which is mainly involved in cell-cell interaction, adhesion, and migration [16]. CD44<sup>+</sup> cancer cells show higher sphere-forming ability and treatment resistance. CD44 is not only a special marker of LSC, but it is also a key regulator of LSC function that is essential for homing of LSCs to microenvironmental niches and for maintaining LSCs in a primitive state.
