We read the comments of Angela Gradilone et al. on our article ''Isolation and characterization of tumorigenic extrahepatic cholangiocarcinoma cells with stem cell-like properties.'' The authors concluded that CD133 and ALDH1 are two other molecular markers of putative cancer stem cells (CSCs) in extrahepatic cholangiocarcinoma (CC). This is an interesting point and very beneficial for further identification of CC stem cells. Actually, this is a big question about how to confirm putative molecular markers for CSCs. Usually, there are two main ways to identify the markers for new type CSCs. First, some normal stem/progenitor cells markers could be used to identify the CSCs, such as leukemia stem cells (CD34þCD38À), brain CSCs (CD133) and breast CSCs (CD24 low/À CD44 þ ESA þ ). Second, some molecular markers that have been confirmed on some CSCs can be used to identify new type CSCs. For example, CD133 first is used to identify brain CSCs, now is the marker for CSCs in liver, colon, pancreas, leukemia and endometrial tumors. CD24/CD44/ESA first are used to identify breast CSCs, individually or in combination, are markers for CSCs in pancreas, prostate, ovarian and colorectal cancers. 1,2 CD133 is very important and extensively used in characterization CSCs. Just as Angela Gradilone et al. mentioned, hepatocellular carcinoma and CC share the same origin. One would expect that CC CSCs express CD133, which is found highly expressed on hepatic progenitor cells. However, whether CD133þ could be added to CD44, CD24 and ESA to further restrict the phenotypic definition of CC stem cells and isolate an even more tumorigenic subset remains to be determined. A recent work has been demonstrated that several different stem/progenitor cell-like subpopulations can exist in leukemia and some solid cancers. Wright et al. 3 showed Brca1 breast tumors contain distinct CD44þ/CD24À and CD133þ cells with CSC characteristics. Hwang-Verslues et al. 4 demonstrated multiple lineages of human breast cancer stem/progenitor cells identified by profiling with stem cell markers. Either CD133 is a further restrict the phenotype or it is a distinct stem/progenitor cell molecular maker, which is really meaningful to detect CD133 expression and identify the CSCslike properties in these subset cells of CC.
In a series of recent studies, ALDH1 has been described to play a crucial role in identification of putative CSCs and/or