Expression of the phosphomimetic Src-S51D mutant rescues formation of actin patches and prevents chromatin breakage in Chk1-deficient cells. (A and B) Cells expressing WT, S51D, or S51A GFP-Src resistant to degradation by siSrc-2 were depleted of the endogenous Src by siSrc-2 and transfected with negative siRNA (control) or siChk1. Broken DNA bridges are indicated by dotted arrows, and the bases of the intercellular canals are indicated by solid arrows. Relative actin patch intensity values are shown. Insets show 1.6× magnification of the canals bases. Bars, 5 µm. (C) Percentage of DNA bridges that appear broken in GFP-positive cells. Error bars show the SD from the mean from three independent experiments. A minimum of 30 cells with chromatin bridges was analyzed per experiment. (D) Actin patches intensity. Relative red fluorescence from A and B is shown, and values in GFP–Src-WT control were set to 1. Error bars show the SD from the mean. n = 30 cells from three independent experiments. ***, P < 0.001 compared with the control. Statistically significant differences were determined by ANOVA and Student’s t test. (E) Model for the role of Chk1 and Src in cytokinesis with chromatin bridges. p, phosphorylation.