SDC4 knockdown reduces the proliferation rate of C2C12 myoblasts by decreasing the progression from G1‐ to S‐phase of the cell cycle. (A) Proliferation of the nontransfected C2C12 cells, and cell lines stably transfected with vectors expressing shRNA against SDC4 or scrambled shRNA was monitored by CellTiter‐Glo assay for 36 h (n = 3 independent experiments/cell line at each time point). (B) Quantification of the frequency of the cells in G1, S or G2/M phases of the cell cycle. Data are reported as means ± SEM, n = 8–15 independent experiments/cell line; 20 000 cells/cell line were counted in each experiment; *P < 0.05, **P < 0.01, ****P < 0.0001. (C) Representative images of the cell cycle analysis of scrambled and shSDC4#1 cells with the fitted curve in pink and respective cell cycle phase terms in green (G1), yellow (S) and blue (G2/M). (D) Representative western blot images show the expression levels of p21, cyclin E and cyclin D1 proteins in the different cell lines. Quantifications of results are shown (n = 4–10 independent experiments). Data are reported as means ± SEM; **P < 0.01, ***P < 0.001, ****P < 0.0001.