Immunohistochemical studies of heart sagittal sections in tbx5 knockdown embryos by cell cycle-related antibodies. A-D: Both cell cycle-related factor CDK2 (red) (A) and P27 (red) (C) expressed mildly in WT embryos but was highly induced in tbx5 knockdown embryos (B, D) at 30 hpf. The nucleus was counterstained with DAPI. The anterior of embryos is to the left. h, heart; MO, tbx5-MO-treated embryos.