Since Id1∼3 are significantly up-regulated in KS lesions compared to adjacent tissue and normal skin and in KMM cells compared to MM cells, up-regulation of Ids by LANA through LANA-Smad1-Id signaling is likely the principal mechanism that KSHV regulates the expression of Ids in KS tumor cells and in KSHV-transformed KMM cells. Here, SMAD1 is linked to Kaposi's sarcoma.