MCM4 and Aarskog-Scott syndrome, X-linked: Less frequently, FGD is caused by inactivating variants of NNT, encoding the nicotinamide nucleotide transhydrogenase (NNT) (10% of cases) [15], STAR, encoding steroidogenic acute regulatory protein (StAR) (5–10% of cases) [16], MCM4, encoding the mini-chromosome maintenance complex component 4 (MCM4) (< 5% of cases) [7], TXNRD2, encoding thioredoxin reductase 2 (TXNRD2) (< 5% of cases) [17], and SGPL1, encoding sphingosine-1-phosphate lyase 1 (SGPL1) (< 5% of cases) [5].