The importance of circulating tumor cells (CTC) is well recognized. However, the biological characteristics of CTC in the bloodstream have not yet been examined in detail, due to the limited number of CTC cell lines currently available. Thirty-nine CTC cell lines were reported by 2021. For successful cell culturing, these CTC cell lines were reviewed. Previous studies on short-term cultures of CTC also analyzed approaches for establishing the long-term culture of CTC. Negative selection, hypoxic conditions, three-dimensional conditions, and careful management are preferable for the long-term culture of CTC. However, the establishment of CTC cell lines is dependent on the specific characteristics of each cell type. Therefore, a method to establish CTC cell lines has not yet been developed. Further efforts are needed to resolve this issue.

Background/Objective: The Empty Spiracles Homeobox 2 (EMX2) gene is a homeobox transcription factor that is critical for the development of the central nervous system and genitourinary system during embryogenesis. EMX2 has been shown to regulate cellular differentiation, migration, and proliferation through its involvement in transcriptional control. Dysregulation of EMX2 expression has been implicated in various pathological conditions, including cancer, but the precise molecular mechanisms underlying EMX2 functions in cancer remain incompletely understood. In this study, we focus on the expression profile and the prognostic significance of EMX2 in esophageal squamous cell carcinoma (ESCC). Methods/Results: The expression levels of EMX2 in clinical ESCC samples varied and appeared to be lower than those in adjacent normal tissues. In addition, EMX2 expression was detected in some of the 20 ESCC cell lines but not in others and was correlated with 5-FU sensitivity. EMX2 expression in ESCC cell lines was strongly associated with colony formation capacity in soft agar, and EMX2 knockdown decreased colony formation. Enforced expression of EMX2 decreased the side population (SP) ratio in FACS analysis but increased colony formation in SP fractions. Although it is a preliminary experiment, xenograft in immunodeficient (NOD) scid mice suggested that the forced expression of EMX2 increased tumorigenic capacity in vivo. A Kaplan–Meyer analysis of patients from whom 20 ESCC cell lines or 18 ESCC tissue samples were obtained indicated that EMX2 expression was a poor prognostic marker. Conclusion: EMX2 has a unique function in ESCC stemness and its expression is the stamped marker of poor prognosis in ESCC patients.