Model: Flp1 eviction by a Sid4-mediated NIMA^Fin1, CK1δ^Hhp1/CK1δ^Hhp2, Chk2^Cds1 relay to boost the impact of Cdk1–cyclin B activation at the SPB. Phosphorylation of T584 by NIMA^Fin1 reduces Sid4 affinity for Ppc89 and supports binding to CK1δ^Hhp1/CK1δ^Hhp2. The CK1δ kinases then phosphorylate T275 and S278 to promote the recruitment of Chk2^Cds1. As we see a BiFc signal between Chk2^Cds1 and Sid4 that is sensitive to competition from Dma1 on the SPB (Fig. 7, C and D; and Fig. S3 C), we assume that the loss of affinity for Ppc89 on the SPB (A) is rapidly followed by a dephosphorylation event after CK1δ kinases have phosphorylated T275 and S278 to create the docking site for Chk2^Cds1. Alternatively, Sid4 anchorage to the SPB is retained while CK1δ kinases phosphorylate T275 and S278. This could be acheived by the phosphorylation of only one Sid4 molecule within a Sid4 dimer (B) or through more complex higher-order associations of Sid4 molecules that await characterization. The T584 phosphatase that would be an essential component in A could equally well operate in the scheme shown in B to remove phosphate from T584 while Chk2^Cds1 is anchored to Sid4 phosphorylated on T275 and S278. Sid4-anchored Chk2^Cds1 phosphorylates Flp1 phosphatase to reduce its affinity for the SPB, thereby lowering antagonism toward Cdk1–cyclin B phosphorylation events on the SPB. The consequence of this cascade is a reduction in the threshold of phosphorylation that must be passed in order to convert the SPB into a mitotic state. The SPB matures from an early G2 state with no potential to invoke mitosis (top; red SPB) to a state with all three kinases in cycles of activation on the SPB (middle; amber) to the mitotic commitment state of Flp1 expulsion from the SPB (bottom; green). We assume that the tipping of the balance between mid-G2 and commitment arises from alterations in the phosphatase activities that dephosphorylate T584, T275, and S278.