**4. CD24 in colon cancer**

CD24 is a glycoprotein located on the external surface of the cellular membrane [16]. It is formed of 27 amino acids, and it has a molecular weight of 24–70 kDa [5, 26]. Its expression was confirmed in normal nervous tissue [16] and in cancers of the colon [5], pancreas [24], breast, and prostate [26]. CD24 is involved in cellular signaling processes, in cellular differentiation, and in proliferation and is being considered a significant marker of cancer stem cells (CSCs) [4, 16, 39]. The mechanism by which CD24 participates in signaling processes seems to be related to mitogen-activated protein kinase (MAPK) and serine/threonine pathway [26].

In colon cancer, CD24 was found to be expressed in a percentage of 50–68% [24, 40]. CD44 is involved in first steps of carcinogenesis and plays an important role in liver metastasis formation [4, 9, 41–43]. Yeo et al. found CD24 a useful diagnostic marker of early colon cancer [39], whereas its expression was higher in malignant polyps than CD24 expression in colon adenomatous lesions.

No correlation was found between CD24 expression in colon cancer and tumor type or degree of differentiation (G) [5, 44]; other authors have highlighted, however, an inverse relation between CD24 expression and tumor size, poor differentiated cancers, and advanced TNM stages [39]. Regarding lymph node involvement and CD24 expression, as association between high CD24 expression and a larger number of lymph nodes involved was reported in some research papers [45] but not in others [5, 24]. In terms of overall survival, CD24 expression was in general associated with worse survival rates [16, 26]; results were not confirmed by other recent research papers [5, 24, 44]. Resistance to chemotherapeutic treatment was also objective by Nosrati et al. [5] probably due to their capacity to induce the epithelial-mesenchymal transition (EMT) mechanism [46]. Moreover, colon cancer stem cells expressing both CD133 and CD24 markers were found to be resistant to chemotherapeutic regiments based on 5-FU [47].

CD24 was highly studied in colon cancer samples, but consistent results have failed to establish its precise role in colon cancer, considering the heterogeneous results observed.
