CD44 and esophageal squamous cell carcinoma: Cluster of differentiation (CD), a class of proteins present on the cell membrane, such as CD14 [15], CD44 [16], and CD47/CD133 [17], can also serve as a surface marker of ESCC stem cells and inhibit the maintenance of tumor stemness by suppressing its expression, making it an independent prognostic factor and a promising therapeutic target for ESCC stem cells.