ID1 and cancer: Our results show that IGF2 secreted by Id1-expressing cancer cells not only activates the tumour microenvironment by inducing fibroblasts to secrete vascular endothelial growth factor (VEGF), but this mechanism also instigates the tumour macroenvironment so that bone marrow cells primed by the presence of Id1-expressing tumours can facilitate tumour growth and distant metastatic colonization.