At the beginning of mitosis, MASTL (GWL, Greatwall kinase) is activated by phosphorylation at several key sites. Many of these sites, including functionally important threonine residues T194, T207 and T741 (corresponding to Xenopus residues T193, T206 and T748), are proline directed, matching CDK1 consensus sequence, and thus probably phosphorylated by CDK1, as shown by in vitro studies (Yu et al. 2006. Blake-Hodek et al. 2012). Phosphorylation of the serine residue S875 (S883 in Xenopus) is implicated as critical for the mitotic function of MASTL (Vigneron et al. 2011) and likely occurs through autophosphorylation (Blake-Hodek et al. 2012). Other kinases, such as PLK1 (Vigneron et al. 2011) and other MASTL phosphorylation sites may also be involved in mitotic activation of MASTL (Yu et al. 2006, Vigneron et al. 2011, Blake-Hodek et al. 2012). Phosphorylation of the serine residue S102 (S101 in Xenopus) is functionally important but the responsible kinase has not been identified (Blake-Hodek et al. 2012).