Chemotaxis of human breast cancer cells in a concentration gradient (0–50 ng/ml) of EGF. (A) First and last images of cells in the gradient region after 5 hours. (B) Directional plot of migration angle in 3–45 ng/ml EGF gradients, grouped in 10° intervals. (C) Cells inside the dotted lines (3–45 ng/ml EGF) were tracked and analyzed. The EGF gradient profile is visualized by FITC-dextran (10 kD) with a similar molecular weight to EGF. Scale bars are 20 μm.