Clarification, affinity capture, virus inactivation, AEX chromatography, CEX chromatography, nano-filtration, UF/DF, etc.  We optimize process parameters and select resin and membranes at this stage. After resin is selected, process parameters will be optimized further. Tests include protein concentration, SEC-HPLC variant (SEC-HPLC), rCE-SDS size variant (rCE-SDS), nrCE-SDS size variant (nrCE-SDS), CEX-HPLC/cIEF charge variant (CEX-HPLC)/ cIEF, residue host cell protein (ELISA), residue protein A (ELISA), residue DNA (qPCR).