Many studies refer to CD44 as a commonly expressed surface marker in different cancer types. The majority of cancer cell lines express high levels of CD44. Differently and in spite of its expression in many different cancer subtypes, the ambiguity on CD24 classification and distribution is still persisting.The conclusions from several studies regarding their expression, role in tumour initiation, and metastasis, and
membrane distribution appear to be different.However, including CD44 and CD24, no marker can be used universally to identify CSCs in various cancers. It is certainly
acceptable that these markers are not expressed in all cancers.We found that the levels of CD44 and CD24 expression show great variation (Figure 1) between cell lines even
in cells of the same cancer subtype. They are engaged with distinct functionalities at different time periods during tumour progression and metastasis [18]. It strongly infers
heterogeneity between and amongst cancer subtypes, which is not fully elucidated and raises a question of credibility regarding their value as CSC surface markers. Despite the extensive study and emerging evidence, significance of CSC markers, their specificity, correlation, and coexistence remain elusive. Also, enormous data from literature is leaving ambiguity even for the same type of cancer subtypes and CSC
markers. Collectively, CSC concept seems to be left more complicated with complex implications yet to be resolved.