Inhibition of the p38 MAP kinase pathway leads to mitotic defects. (a, b). HeLa cells were synchronized with the double thymidine block, released for 6 h, treated with SB203580 for additional 6 h and stained with a phospho-H3 antibody. Scale bar, 5 µm. (c) Histogram quantifying the chromosome lagging phenotype after SB203580 treatment. (d) Cells were synchronized with the double thymidine block, released for 8 h, treated with SB203580 for additional 2, 4 or 6 h, and stained with a phospho-H3 antibody.