Selections for glycosylation mutants. Cytotoxic lectins or agents that bind to specific sugar residues select for resistant cells (left). Screen for mutants using replica plating. Colonies on plastic are transferred to discs and screened for defects in incorporation of radioactive precursors, binding to lectins and antibodies, or direct enzymatic assay. Mutants are colonies lacking a strong signal (middle). HAP1 human haploid cells mutagenized by infection with a gene trap retrovirus and selected for resistance to a Lassa pseudovirus (which needs glycosylated α-DG to infect cells) or immunodepleted for glycosylated α-DG (right).