Before mounting, crystals were cryo-protected with mother liquor supplemented with an additional 15% ethylene glycol and vitrified in liquid nitrogen. The CDK12715–1052/cyclin K11–267 complex was buffered in 50 mM HEPES pH 7.5, 300 mM NaCl, 1 mM TCEP, 10 mM DTT, 5 mM L-...