This time point was selected because data from Fig. 5b showed that the 2-week duration was sufficient for the Id1-expressing tumours to condition the bone marrow and enrich it with VEGFR1+ cells, and yet the tumours of the three different groups were still of comparable volume (Fig. 5g) so the tumour size of the ‘donor' mice would not be a confounding variable in the following experiments. This evidence concerns the gene FLT1 and neoplasm.