**6. CD166 or ALCAM**

CD166, also called activated leukocyte cell adhesion molecule (ALCAM), is a 110 kDa, transmembrane type-1 glycoprotein used for colon cancer stem cell (CCSC) identification [3, 61, 62]. Providing the leukocyte receptor function, CD166 expression was identified in both normal and colonic tissue, in the latter cases the expression being superior [3, 63]. CD166 expression in colon cancer varies between 58.6 and 76% [64, 65] and is higher in colonic adenomas [66], suggesting its involvement in colon carcinogenesis. Due to its adhesive properties, CD166 is considered to be involved in colon cancer tumor growth [62]. CD166 expression was also confirmed in pancreatic, esophageal and gastric, prostate, melanoma, and breast cancers [63].

Expression of CD166 in colon cancer was studied in relation to tumor stage [61, 64, 65], lymph node involvement [61, 64], or degree of cellular differentiation (G) [61], but even if overexpression was confirmed, no statistic significant correlation was found. Regarding overall survival of colon cancer patients,

overexpression of CD166 failed to predict its outcome. Some literature studies found a worse overall survival in colon cancer cases characterized by high CD166 expression [64]. Levin et al. found that even the survival was reduced by 15 months for patients who presented colon tumors characterized by high CD166 expression compared with tumors with low or absent CD166 expression [67]. Other studies could not establish the prognostic relation of CD166 in colon cancer patients [65].

Limited number of studies analyzed CD166 expression in colon cancer patients, and existing results are inconclusive. Therefore, the role of CD166 in colon cancer remains unclear.
