Association of CD44 with highly aggressive breast tumor subtypes or highly invasive breast cancer cells. Oncomine analysis of CD44 expression in breast cancer demonstrated significantly higher CD44 mRNA expression in (A) basal-like breast cancer tumors in comparison with luminal tumors and in (B) basal-like breast cancer cells lines in comparison with luminal-like cell lines (P = 1.90E-6). (C) Bar graph showing elevated CD44 mRNA transcript levels present in the MDA-MB-231Hi cells compared with the MDA-MB-231 parental cells (3.26-fold increase; n = 5; P < 0.01). (D) Representative immunoblot showing upregulation of CD44s in MDA-MB-231Hi cells relative to the MDA-MB-231 parental cell line. The blots were reprobed with β-tubulin as a loading control. (E) Bar graph illustrating the elevated invasive capacity of the MDA-MB-231Hi cells (32.28% ± 2.33% of invaded cells/total number of cells; n = 3; P < 0.05) through Matrigel relative to the MDA-MB-231 cells (18.23% ± 2.071%). (F) CD44 knockdown in MDA-MB-231Hi reduced the cell-invasion index over a 36-hour period through Matrigel from 0.89 ± 0.07 to 0.24 ± 0.11 (n = 3; P < 0.01). Statistically significant differences were determined by using a Student two-tailed t test (*P < 0.05; **P < 0.01).