The transcription factor caudal-type homeobox 1 (CDX1) is a key regulator of differentiation in the normal colon and in colorectal cancer (CRC). CDX1 activates the expression of enterocyte genes, but it is not clear how the concomitant silencing of stem cell genes is achieved. MicroRNAs (miRNAs) are important mediators of gene repression and have been implicated in tumor suppression and carcinogenesis, but the roles of miRNAs in differentiation, particularly in CRC, remain poorly understood. Here, we identified microRNA-215 (miR-215) as a direct transcriptional target of CDX1 by using highthroughput small RNA sequencing to profile miRNA expression in two pairs of CRC cell lines: CDX1-low HCT116 and HCT116 with stable CDX1 overexpression, and CDX1-high LS174T and LS174T with stable CDX1 knockdown. Validation of candidate miRNAs identified by RNA-seq in a larger cell-line panel revealed miR-215 to be most significantly correlated with CDX1 expression. Quantitative ChIP-PCR and promoter luciferase assays confirmed that CDX1 directly activates miR-215 transcription. miR-215 expression is depleted in FACS-enriched cancer stem cells compared with unsorted samples. Overexpression of miR-215 in poorly differentiated cell lines causes a decrease in clonogenicity, whereas miR-215 knockdown increases clonogenicity and impairs differentiation in CDX1-high cell lines. We identified the genome-wide targets of miR-215 and found that miR-215 mediates the repression of cell cycle and stemness genes downstream of CDX1. In particular, the miR-215 target gene BMI1 has been shown to promote stemness and self-renewal and to vary inversely with CDX1. Our work situates miR-215 as a link between CDX1 expression and BMI1 repression that governs differentiation in CRC.T he caudal-type homeobox 1 (CDX1) transcription factor controls enterocyte differentiation in the colon, where its expression is excluded from the crypt-base stem cell compartment. CDX1 is also central to the capacity of a colorectal cancer (CRC) cell line to differentiate, and it is a negative marker of CRC stem cells (1-3). In 19% of CRC cell lines assayed in a large study, CDX1 expression was completely lost due to promoter methylation and was down-regulated in a further 13% as a result of hemimethylation of the promoter (4). Comparison of CDX1 expression in colorectal adenocarcinoma versus matched normal tissue showed downregulation of CDX1 in 73% of tumors, which was also attributable to promoter methylation (5). Expression of CDX1 also correlates inversely with that of the polycomb complex protein BMI1, which is necessary for the maintenance of quiescent injury-inducible stem cells in the normal crypt and is expressed in cancer stem cells (2, 6-8). The mechanism underlying this inverse correlation has not yet been elucidated.In addition to these correlative data, CDX1 has also been shown to promote directly the expression of structural proteins important for epithelial differentiation including cytokeratin 20 (KRT20) (1), villin (VIL) (9), and FABP1 (10). Introduction...