APEX2 and melanoma: To evaluate the ability of S9AF to specifically label interactors of Siglec‐9 through the peroxidase activity of APEX2, we incubated purified S9AF with the human melanoma cell line A375, which was reported to express a high level of Sigelc‐9 ligands.[9] After the addition of substrates for APEX2 including biotin‐phenol and H2O2 to the cells, biotinylated proteins in cell lysates were detected by western blot using the HRP‐conjugated streptavidin (Figure 1C).