P4 inhibits neuroblastoma growth by inducing apoptosis. (A) Representative photomicrographs (40x) of immunohistochemistry and quantification of cleaved caspase-3-positive cells in tumor sections. The arrow shows the brown-stained cleaved caspase-3-positive cells. TUNEL assay was performed to confirm apoptotic effect of P4 in the neuroblastoma tumor. (B) Representative photomicrographs (20x) and quantification of TUNEL-positive cells in different groups. Data are expressed as means ± SD. Significant difference: *P < 0.05 compared with vehicle group. P50, P4 (50 mg/kg); P100, P4 (100 mg/kg).