SCID mice were xenografted subcutaneously with 10 × 106HCC-1599 cells on both sides. One week after transplantation 10 animals/experimental group were treated intraperitoneally with vehicle (DMSO) or two doses of ATRA (7.5 and 15.0 mg/kg) once/day, 5 days a week for a total of 24 days. At the end of this period, treatment was discontinued until sacrifice.The size of the tumors was determined with a caliper and the volume plotted. Each point is the mean ± SE of 20 tumors. *Significantly lower than the corresponding vehicle-treated group (P-value < 0.05, Student's t-test). **Significantly lower than the corresponding vehicle-treated group (P-value < 0.01, Student's t-test). 