42 Background: MicroRNAs (miRNAs) are key regulators of cancer stem cell (CSC) functions, including self-renewal and differentiation. However, little is known about miRNAs that regulate such properties in human colorectal cancer. Methods: We compared the expression levels of 754 miRNAs between paired samples of EpCAM+/CD44+ cancer cells (enriched in CSCs) and EpCAM+/CD44neg cancer cells (non-tumorigenic cancer cells (NTCs)) sorted in parallel from human primary colorectal cancers by multiplex semi-quantitative PCR. Results: Comparison of the expression levels of miRNAs in the CSCs and NTCs resulted in the identification of 10 miRNAs upregulated (miR-221, miR-218, miR-500 and miR-195) or downregulated (miR-185, miR-19a, miR-337, miR-10a, miR-21 and miR-24) in CSCs compared to NTCs. Among them, miR-221 was most highly expressed in CSCs; and its expression level was very low in NTCs and normal colon epithelial cells (n=6, p<0.05). Kaplan-Myer estimates and univariate and multivariate analyses showed that higher miR-221 expression was an independent prognosis factor for poorer outcome. Constitutive overexpression of miR-221 enhanced organoid-forming capacity of both conventional colorectal carcinoma cell lines and patient-derived xenografts (PDX) in vitro. Importantly, constitutive downregulation of miR-221 suppressed organoid-forming capacity in vitro and substantially reduced the tumorigenic capacity of CSC populations from PDX lines in vivo. miR-221 directly targeted Quaking (QKI) 5, the most abundant splicing isoform of the human QKI gene, and suppressed its protein level. Knockdown of miR-221 suppressed the clonogenicity of colorectal CSCs in vitro in a QKI-expression dependent manner and ability to form tumors in vivo. Conclusions: Our study reveals a mechanistic link between miR-221 and QKI and highlights their key role in regulating CSC properties in human colorectal cancer and proposes that miR-221 will be a promising marker and a specific therapeutic target for human colorectal CSCs.