Overexpression of p21 partly reversed the effect of knockdown of SMAD3 in lung adenocarcinoma cells. (A, B) Cell cycle analysis demonstrated that overexpression of p21 significantly decreased the percentage of G2/M phase cells in A549 cells infected with shRNA targeting SMAD3 by flow cytometry. (C) Overexpression of p21 reversed the inhibition of proliferation caused by knockdown of SMAD3 in A549 cells. (D, E) Analysis of clone formation assay by single target multi-shot model curve fitting demonstrated that the number of cell colonies was significantly increased by overexpression of p21 in A549 cells infected with shRNA targeting SMAD3 at the same dose of irradiation. shRNA, short hairpin RNAs. Data are presented as mean ± SEM. *P < 0.05, **P < 0.01, and ***P < 0.001. SEM, standard error of mean.