The 6xHis-tagged mouse DHFR was expressed at the indicated levels in E. coli, purified from 3 ml cultures using the Ni-NTA Spin Kit under denaturing conditions, and eluted in buffer at pH 5.9. Fractions were visualized by Coomassie staining after SDS-PAGE. 5 µl (of each 200 µl eluate) was loaded. 1: cell lysate; 2: flow-through 3: first eluate; 4: second eluate.