Assaying serum PLP is particularly helpful when hypophosphatasaemia has an explanation other than hypophosphatasia, because an elevated level might be expected exclusively in hypophosphatasia as the activities of all TNSALP isoforms, not just from bone, are low, and the other causes of hypophosphatasaemia seem to particularly suppress bone TNSALP activity (1,6). The gene discussed is ALPL; the disease is hypophosphatasia.