For seizures that respond to vitamin B6 treatment but no biochemical or molecular markers of PDE are found, further diagnostics should be performed for the conditions broadly discussed in the differential diagnosis section, such as pyridoxal phosphate responsive epileptic encephalopathy, caused by deficiency of pyridoxamine 5′-phosphate oxidase (PNPO); tissue-nonspecific alkaline phosphatase (TNSALP) deficiency; familial hyperphosphatasia (PIGV deficiency) and hyperprolinemia type II [1,15,17,30,31,32]. Here, PNPO is linked to hyperprolinemia type 2.