Effect of Jab1 short interfering RNA (siRNA) knockdown on p27 levels with and without epidermal growth factor (EGF) treatment in MDA-MB-231 cells. (a) Western blot analysis of cell lysates collected following the indicated treatments. Relative p27 band intensities were calculated by densitometry and normalized to the loading control (GAPDH). (b) The percentage change in p27 expression for EGF-treated cells relative to untreated controls. Data shown are representative of several independent experiments, demonstrating that knockdown of Jab1 abrogates EGF-induced downregulation of p27. Bars represent mean ± standard deviation. Statistical analysis was by t test (*P < 0.0001). All EGF treatments were at 50 ng/mL for 4 hours. EGFR, epidermal growth factor receptor; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; Jab1, c-Jun activation domain-binding protein-1; pEGFR, phosphorylated epidermal growth factor receptor.