Chemo-resistance, overexpression of miR-21 and colonosphere formation in colon cancer HCT-116 cells, all of which are associated with increased miR-21 levels, lead to down-regulation of miR-145, and miR-145 and miR-21 negatively regulate each other in chemo-resistant (CR) colon cancer cells. (A) qRT-PCR showing up-regulation of mature miR-21 and down-regulation of mature miR-145 in CR HCT-116 cells when compared with the parental HCT-116 cells. (B) qRT-PCR showing over-expression of miR-21 (stably transfected cells) leads to decreased miR-145 compared to the vector-transfected control. (C) qRT-PCR showing down-regulation of miR-145 and up-regulation of mature miR-21 in colonospheres formed by parental HCT-116 cells. (D) qRT-PCR showing relative expression of miR-21 and miR-145 in CR HT-29 cells at 48 h following transfection of mature miR-145 or anti-miR-21. All the data represent means ± SD of three independent experiments, *P < 0.001, compared to the control.