SBC2 protein (40 µM), SBC2 C111S, SBC4, SBC6 or CISH were incubated with an equimolar concentration of MN551 in 50 mM Tris, pH 8.0, 50 mM NaCl, 1 mM TCEP. At de8ined time points 20 µl samples were removed and the protein precipitated by addition of 80 µl methanol. The precipitated protein was pelleted by centrifugation, washed with 500 µl methanol and resuspended in an aqueous solution of 15% acetonitrile and 1% TFA. Samples were separated by HPLC on a C3 column using a 10 to 75% gradient of acetonitrile and analysed using an Agilent 6130 quadrupole MS. Spectra were deconvoluted and integrated using Agilent LC/MSD ChemStation.