All the animal experiments were performed according to the guidelines, following a protocol approved by the Institutional Animal Care and Use Committee (IACUC). ICR-SCID mice, 6–8 week-old, were purchased from Jackson laboratory (Bar Harbor, ME); athymic nu/nu mice, 6–8 week-old, were purchased from Envigo (Indianapolis, IN) and housed at the Research Animal Resource Center (RARC) of Memorial Sloan-Kettering Cancer Center. The facility is approved by the American Association for Accreditation of Laboratory Animal Care and is maintained in accordance with the regulations and standards of the United States Department of Agriculture and the Department of Health and Human Services, NIH.