PLK1 and cancer: We examined the crystal-packing interactions formed by the polo-box domain (PBD) of polo-like kinase 1 (Plk1), a validated anti-cancer target,13 whose phosphorylation-dependent protein–protein interactions are crucial for successful progression of the cell through mitosis.14, 15 The PBD has been well characterized crystallographically, both unliganded and in complex with various phosphopeptides: we have obtained ten new crystal forms, which supplement nine already in the public domain.16–19 Together, these provided extensive data for crystal-packing analysis.