**4.12. HOXC9**

*HOXC9* is a transcription factor expressed in blood vessels in mice [81] and in the cardinal vein of zebrafish [82]. Kroll's group investigated this transcription factor in human vascular endothelial cells and zebrafish, and they observed that this protein was a negative regulator of circulating endothelial cells. They found that HoxC9 was highly expressed in resting endothelial cells; however, its expression was downregulated under hypoxic conditions, and overexpression of this factor inhibited endothelial migration, tube formation, and endothelial cell proliferation by targeting IL-8 transcription [82]. Finally, using a zebrafish model, they observed in vivo that HoxC9 overexpression inhibited the development of their vascular structure; this defect was rescued with exogenous IL-8. This data suggests that HoxC9 plays a negative role in the induction of endothelial cell growth by inhibiting IL-8 production [81, 82].
