For PADI4 immobilisation, bio-His-PADI4 or bio-His-PADI4_C645A were incubated with magnetic streptavidin beads (Invitrogen) (4 °C, 15 min to an immobilisation level of 0.9 pmol/μL beads) immediately before use in the selection. Biotin was added to cap unreacted streptavidin sites (25 μM final, 4 °C, 15 min). Beads were washed 3 x 1 bead volume selection buffer and left on ice for use in the selection. Negative beads were prepared similarly except that only selection buffer or selection buffer plus biotin (25 μM) were added to beads and following washing these two variants were mixed.