POR (cytochrome P450 oxidoreductase) is a ubiquitously expressed gene encoding an electron donor to all microsomal P450 enzymes and several non-P450 enzymes. POR mutations cause an autosomal recessive disorder characterized by skeletal dysplasia, adrenal dysfunction, and disorders of sex development. Although recent studies have indicated the presence of a CpG-rich region characteristic of housekeeping genes around the untranslated exon 1 (exon 1U) and a tropic effect of thyroid hormone on POR expression via thyroid hormone receptor-β, detailed regulatory mechanisms for the POR expression remain to be clarified.Our objective was to report a pivotal element of the proximal promoter of POR.We first studied three patients (cases 1-3) with POR deficiency due to compound heterozygosity with an p.R457H mutation and transcription failure of an apparently normal allele, by oligoarray comparative genomic hybridization and serial direct sequencing of the deletion fusion points. Consequently, a 2,487-bp microdeletion involving exon 1U was identified in case 1 and an identical 49,604-bp deletion involving exon 1U and exon 1 was found in cases 2 and 3. We next analyzed the 2,487-bp region commonly deleted in cases 1-3 by in silico analysis, DNA binding analysis, luciferase assays, and methylation analysis. The results showed a critical function of the evolutionally conserved SP1 binding sites just upstream of exon 1U, especially the binding site at the position -26/-17, in the transcription of POR.The results suggest that the SP1 binding sites constitute an essential element of the POR proximal promoter.