DDC and aromatic L-amino acid decarboxylase deficiency: It is known that the absence of functional AADC leads to altered levels of dopamine in plasma and cerebrospinal fluid (CSF) of AADC deficiency patients [2], who in addition present also low dopamine catabolite levels (such as 3‐methoxy tyramine (3‐MT), 3,4‐dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA)) and high amounts of the l‐dopa derivative 3‐O‐methyldopa (3‐OMD), suggested to be supportive as a biomarker for AADC deficiency [3, 5].