Growth-inhibited tumors exhibit increased numbers of intra-tumoral activated T cells and decreased expression of Ccl17,Ccl22, and Foxp3. Marked reduction in Treg proliferation and tumor infiltration coincide with G1 arrest in tumor infiltrated Siah2−/− Tregs in vivo or following T cell stimulation in culture, attributed to elevated expression of the cyclin-dependent kinase inhibitor p27, a Siah2 substrate. The gene discussed is FOXP3; the disease is neoplasm.