Depending on deficient enzymatic activity, D-BPD is classified into four types: type I—defined by dehydrogenase and hydratase deficiency; type II—associated with only hydratase deficiency; type III—defined by dehydrogenase deficiency; type IV—described by McMillan [5], with a less severe phenotype compared to types I, II, and III, similar to Perrault syndrome (PRLTS) (OMIM #233400), another rare disease associated with mutations of the HSD17B4 gene [3,5,6,7,12,16]. The gene discussed is HSD17B4; the disease is Perrault syndrome.