We analyzed CtIP immunoprecipitated and purified from stably transfected HEK293T cells with or without in vitro incubation with recombinant Plk3. All observed phosphorylation sites are shown. Statistical analysis (Ascore calculation) was performed at the Taplin Mass Spectrometry Facility (Harvard Medical School; Beausoleil et al., 2006). The sites were classified as P = phosphorylation (Ascore ≥ 19), U = unlikely phosphorylation (Ascore 0–18), and N = no phosphorylation. CtIP phosphorylation (Ascore ≥ 19) at S327 and T847 was detected with, but not without, Plk3 incubation. 17 additional CtIP sites were phosphorylated with Plk3 but not phosphorylated without Plk3 incubation. Note the high basal CtIP phosphorylation level without Plk3 incubation.