In the normal receptor recycling pathway (left), the receptor PEX5 inserts an N-terminal segment into the ligase pore, positioning a conserved cysteine for monoubiquitination by the RING finger of PEX2 (RF2). A region of PEX5 is in the background behind PEX12 and is indicated by a broken line. In the RADAR pathway (right), which is activated when normal recycling is compromised, the N-terminal segment of PEX5 moves sideways to position a lysine residue for polyubiquitination by RF10. A region of PEX5 is in the background behind PEX10 and is indicated by a broken line.