Polo-like kinase 1 has hundreds of substrates and multiple functions that operate within the ∼60 min of mitosis. This paper describes a chemical-genetic system that allows particular substrates to be “toggled” into or out of chemical control using engineered phosphoacceptor selectivity. Kif2b, a known substrate of Plk1 that regulates chromosome alignment was evaluated. Toggling Ser to Thr on Kif2b places these phosphorylation sites under reversible chemical control. Thus, it is demonstrated the ability to chemically control a single substrate by a genetic Ser/Thr toggle.