Causative genes of isolated FGD are the ACTH receptor (MC2R; also formerly defined as FGD 1) (56), the melanocortin receptor-associated protein (MRAP; also formerly defined as FGD 2) (46), the DAX1 transcription factor (NR0B1) (57), nicotinamide nucleotide transhydrogenase (NNT) (58–60), and mitochondrial thioredoxin reductase (TXNRD2) (61). The gene discussed is NNT; the disease is Aarskog-Scott syndrome, X-linked.