Purpose The aim of this study was to assess the expressions of CD44 and CD133 in colorectal cancer tissue by using immunohistochemical staining and to analyze the clinical significance of the expressions related to other clinicopathological data and survival results. expression was lower in cases with elevated CA 19-9 serum levels (P = 0.028) and advanced HMMR T stage (P = 0.038). Multivariate analysis proved that low expression of CD44 was an independent prognosis factor for short disease-free survival (P = 0.028). Conclusion Low CD44 expression was correlated with increased tumor recurrence and short disease-free survival, and low CD133 expression was associated with advanced tumor stage. We suggest that further studies be performed to 196808-24-9 IC50 evaluate whether the immunohistochemical method for determining the CD44 and the CD133 expressions is appropriate for exploring cancer stem-cell biology in patients with colorectal cancer. Keywords: Colorectal neoplasms, CD40 antigens, CD133 antigen, Stem cell INTRODUCTION Colorectal cancer is the third most common cancer in men and the second most common cancer in women worldwide . The incidence of colorectal cancer in East Asian countries, including Japan and Korea, has increased sharply, probably due to a Westernized diet and lifestyle . Mortality from colorectal cancer accounts for 8% of all cancer deaths, and colorectal cancer is the fourth most common cause of death from cancer . Recently, colorectal cancer mortality has decreased in developed countries owing to better treatments and early detection . New chemotherapeutic agents and targeted therapies have shown promising results of improving survival in colorectal cancer patients [4,5]. However, more than 30% of stage III colon cancer patients suffer a recurrence even though they may have received a curative resection and adjuvant chemotherapy with oxaliplatin . The median progression-free survival time in metastatic colorectal cancer patients is only 8.9 months even after treatment with cetuximab and chemotherapy . Tumor recurrence and chemoresistance are the main problems that need to be solved if survival in cancer patients is to be prolonged. Recently, cancer stem cells (CSCs) have received attention due to their role in cancer initiation, progression, and metastasis . Their ability of self-renewal, unlimited proliferation, and multipotency are considered cancer stem-cell phenotypes, and they seem to be responsible for local relapse and metastasis by inducing resistance against traditional drug therapy . Specific cell surface markers for CSCs are needed for identifying and sorting the CSCs. Several markers for CSCs have been investigated and proposed in colorectal cancer, and CD44 and CD133 have been the most frequently researched and 196808-24-9 IC50 are thought to be the most likely markers for colorectal CSCs [8,9,10]. In this study, we evaluated the expressions of CD44 and CD133 in colorectal cancer tissue by 196808-24-9 IC50 using the immunohistochemical staining method, and we analyzed the clinical significance of the results. METHODS Patients and clinicopathological data One hundred sixty-two patients with a biopsy-proven colorectal adenocarcinoma who were operated on between January 1998 and August 2004 were enrolled in this study. Patients’ data recorded in our colorectal cancer database were analyzed. The following clinicopathological factors were selected and evaluated: gender, age, location of tumor (right colon, left colon, or rectum), tumor size, tumor’s gross appearance, carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA 19-9), TNM stage (American Joint Committee on Cancer. 7th ed.), tumor differentiation, and recurrence of tumor. Immunohistochemical staining method Staining for CD44 and CD133 was performed on primary colorectal 196808-24-9 IC50 cancer tissue, metastatic lymph nodes, 196808-24-9 IC50 and synchronous and metachronous metastatic tumor tissues if available. Tissue arrays were prepared by consigning them to the SuperBio Chips, Co. (Seoul, Korea). Tissue array blocks were sectioned to be 4 m in thickness, and immunohistochemical staining was performed using a Bond polymer detection kit and Bond-max autostainer (Leica.