Figure of the publication by Mills and collaborators showing the biochemical effect of the various PNPO variants found in patients of their study. This assay measures the capacity of the PNPO enzyme to convert pyridoxine (vitamin B6) to pyridoxal phosphate (PLP). All variants show a reduction of PNPO activity. However, some PNPO variants such as R116Q only have a small effect, which is consistent with the late onset phenotypes in patients carrying these variants (figure adapted from Mills et al., 2014 under a Creative Commons Attribution Non-Commercial License)