PCBD1 encodes pterin-4 alpha-carbinolamine dehydratase 1, a bifunctional protein with roles in tetrahydrobiopterin (BH4) metabolism and transcriptional regulation. As a metabolic enzyme, PCBD1 catalyzes the conversion of 4-alpha-hydroxytetrahydrobiopterin to BH4, an essential cofactor for phenylalanine hydroxylase and monoamine neurotransmitter synthesis 1. Loss-of-function PCBD1 mutations cause autosomal recessive hyperphenylalaninemia with BH4 deficiency (HPABH4D), presenting with elevated plasma phenylalanine and potential neurotransmitter deficiencies 1. Beyond its metabolic function, PCBD1 acts as a coactivator for hepatocyte nuclear factor-1B (HNF1B), a transcription factor critical for kidney and pancreatic function 2. PCBD1 facilitates HNF1B dimerization and enhances transcription of FXYD2, which encodes a protein essential for magnesium reabsorption in the distal convoluted tubule 2. Consequently, PCBD1 mutations cause secondary complications beyond neonatal presentations, including renal magnesium wasting and hypomagnesemia 2, along with maturity-onset diabetes of the young (MODY)-like diabetes 23. Population-based studies suggest PCBD1 is an inconclusive MODY-causal gene with lower penetrance compared to established MODY genes 4. These findings establish PCBD1 as a critical regulator of both amino acid and mineral homeostasis.