OV6, which was originally classified as a marker of hepatic progenitor cells, was also identified in a subpopulation of cells with a high ability to form tumors in vivo and with substantial resistance to standard chemotherapy [
7]. OV6+ cells also exhibited strong invasive and metastatic potential both in vitro and in vivo [
56]. CD13 was identified as a novel cell surface marker for CSCs by Haraguchi et al.[
9]. They found that CD13+ HCC cells were CSCs enriched in a side population of cells from several HCC cell lines, predominated in the G0 phase of the cell cycle, and initiated tumor formation. Mechanistically, they found that CD13 protects cells from apoptosis via the ROS scavenger pathway. CD24, a mucin-like cell surface glycoprotein, was found to be a functional liver CSC marker that drives CSC genesis through STAT3-mediated NANOG regulation [
10]. Xu et al. reported that DLK1+ HCC cells have characteristics similar to those of CSCs and showed higher levels of chemoresistance, colony formation, spheroid colony formation, and in vivo tumorigenicity than DLK1− cells [
11]. Zhao et al. reported that α2δ1 is a functional liver CSC marker identified using a monoclonal antibody against recurrent HCC, 1B50-1, which binds to the calcium channel α2δ1 subunit. The role of α2δ1 isoform 5 in liver CSCs is related to its regulation of calcium influx through
l- and N-type voltage-gated calcium channels [
12]. ICAM1, which was reported as a marker of CSCs and circulating tumor cells in humans and mice, is regulated by the stem cell transcription factor NANOG [
13]. Lee et al. found that CD47 is expressed in liver CSCs, which contributes to tumor initiation, self-renewal, and metastasis, and significantly affects the clinical outcome of patients. In addition, they found that CD47+ HCC cells regulate liver CSCs through the cathepsin S/protease-activated receptor 2 paracrine loop [
14]. Lgr5, which is also known as a marker of liver cells following damage [
35], was reported to be a potential CSC marker showing high tumorigenicity and resistance to chemotherapeutic agents [
15]. Most recently, keratin19, also known as CK19, was verified as a CSC marker of HCC associated with EMT and TGFβ/SMAD signaling [
16]. Using a functional approach, Muramatsu et al. identified liver CSCs by visualization system of proteasome activity and ROS level. They demonstrated that HCC subpopulation with low proteasome activity/low ROS level shows liver CSC properties and tumorigenicity in vivo. They further indicated that these liver CSCs facilitate the migration of macrophages to organize their niche, and induces metastasis by the recruitment of macrophage [
57].