ACTR2 and neoplasm: Since RAC1, upstream of cortical branched actin, is also an oncoprotein activated by the hotspot missense mutation P29S in melanomas,23 we collected cell lines that specifically harboured oncogenic forms of RAC proteins.24–26 Contrary to randomly picked tumour cell lines, which were insensitive to Arp2/3 inhibition, all the cell lines that contained an active RAC1 protein stopped cycling upon Arp2/3 inhibition (Fig. 8a).