QDPR and Hyperphenylalaninemia: RLs were set up in the form of a consortium of two laboratories for two analyses, namely for (1) the assessment of α-aminoadipic semialdehyde in urine and δ1-piperideine-6-carboxylate in plasma and urine (mainly used in the case of pyridoxine-dependent seizures), and (2) the assessment of pterins in urine and of the 6,7-dihydropteridine reductase activity in dry blood spots (used in the context of hyperphenylalaninemia).