Prior to anaphase onset, sister-chromatids are held together by cohesin complexes. PLK1-dependent phosphorylation of the cohesin subunit STAG2 (SA2) (Hauf et al. 2005) promotes dissociation of cohesins from chromosomal arms in prometaphase (Hauf et al. 2001). Besides phosphorylating STAG2, PLK1 also phosphorylates RAD21 cohesin subunit, but the phosphorylation of RAD21 is not required for the dissociation of cohesin from chromosomal arms in early mitosis (Hauf et al. 2005). There are several potential PLK1 phosphorylation sites in STAG2 and RAD21, but the exact positions of in vivo phosphorylation of STAG2 and RAD21 by PLK1 have not been explicitly established (Hauf et al. 2005). It is likely that the phosphorylation of cohesin-bound CDCA5 (Sororin) by CDK1 creates a docking site for PLK1 at threonine T159 of CDCA5, thus enabling PLK1 to phosphorylate cohesin subunits (Zhang et al. 2011).