ID1 and osteosarcoma: Consistent with gene specific interactions, the Id proteins also exhibit isoform specific functions such as modulation of breast cancer 1, early onset (BRCA1) promoter activity by Id4 [15,16], localization of Id1 to the centrosomes [17] leading to accumulation of cells with abnormal centrosome number and induction of apoptosis by Id2 in myeloid precursors, osteosarcoma [18] and neuronal cells [19] by an HLH independent mechanism.