This fraction of Mad1 does not colocalize with nucleoli (Supplementary Fig. 1a–b), but does show substantial colocalization with Myc and HA tagged SUMO1 and SUMO2 as well as endogenous SUMO1 in MDA-MB-231 breast cancer cells and in HeLa cervical cancer cells (Fig. 1a–c, f, Supplementary Fig. 1b–e). Here, SUMO2 is linked to breast carcinoma.