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POR
cytochrome p450 oxidoreductase
POR is located on the long (q) arm of chromosome 7, at band 7q11.23. Arm ratio per GRCh38 - banding schematic.
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Overview
The POR gene provides instructions for producing an enzyme called cytochrome P450 oxidoreductase, which acts as an essential electron donor enabling more than 50 different cytochrome P450 enzymes to carry out their biochemical functions. These partner enzymes perform critical tasks including the production of steroid hormones - testosterone, oestrogen, corticosteroids, and aldosterone - as well as generating cholesterol molecules that play vital roles in skeletal growth and development. The oxidoreductase enzyme also contributes significantly to how the liver processes medications and other ingested compounds, transferring electrons to cytochrome P450 proteins that chemically modify these substances.
Situated on chromosome 7, POR falls within pharmacogenomics gene classifications because sequence differences may influence how individuals metabolise specific pharmaceuticals. When both copies of the gene carry pathogenic variants, the resulting cytochrome P450 oxidoreductase deficiency disrupts hormone-dependent developmental processes and bone formation. The gene's importance spans both internal metabolic pathways involving steroid synthesis and external compound processing, making it clinically relevant for both metabolic diagnostics and individualised medicine strategies.
What the gene does
Cytochrome P450 oxidoreductase serves as the required electron transfer partner for microsomal cytochrome P450 proteins, shuttling electrons from NADPH to activate their catalytic functions. This electron delivery system is essential for converting cholesterol into pregnenolone, the starting material for all steroid hormone production, and for the subsequent chemical modifications that yield cortisol, aldosterone, and sex hormones. When the oxidoreductase enzyme fails to function properly, cytochrome P450 proteins cannot complete these biosynthetic pathways, resulting in disrupted hormonal signalling during development both before and after birth.
In addition to its role in steroid production, the enzyme assists liver-based cytochrome P450 proteins that chemically modify ingested medications during phase I metabolism, thereby affecting how quickly therapeutic agents are cleared from the body. The oxidoreductase anchors itself to endoplasmic reticulum membranes, where it can interact sequentially with different P450 enzyme partners. Evidence suggests that naturally occurring sequence differences in POR may account for some of the variation between individuals in how well drugs work and how likely they are to cause unwanted effects, though researchers are still investigating the clinical importance of common genetic variants. The enzyme's ability to support many different substrate-processing reactions highlights its central position in maintaining metabolic balance and clearing foreign compounds.
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Chromosome location
POR resides at chromosomal band 7q11.23 on the long arm of chromosome 7. This locus lies within a region occasionally subject to genomic rearrangements, though POR itself is not typically involved in the microdeletion syndromes associated with neighbouring genes. The gene spans multiple exons encoding a 677-amino-acid protein, with regulatory elements that govern tissue-specific expression levels in liver, adrenal glands, and gonads.
Protein structure
The 677-amino-acid cytochrome P450 oxidoreductase comprises distinct functional modules that together facilitate electron transfer. The Flavodoxin-like domain, spanning amino acids 80-224, binds flavin mononucleotide (FMN) and accepts electrons from the FAD cofactor for transfer to cytochrome P450 partners. Downstream, the FAD-binding FR-type domain occupies residues 279-521, coordinating flavin adenine dinucleotide and accepting electrons directly from NADPH. These two flavin-binding regions work in concert, with electrons flowing from NADPH to FAD, then to FMN, and finally to the haem iron of cytochrome P450 enzymes. The spatial arrangement of these domains allows conformational shifts that bring the FMN module into proximity with various P450 active sites, enabling the oxidoreductase to serve multiple enzymatic partners sequentially.
Key variants
More than 50 pathogenic variants in POR have been identified in individuals with cytochrome P450 oxidoreductase deficiency. These genetic alterations impair the enzyme's capacity to transfer electrons efficiently, thereby reducing the catalytic performance of the cytochrome P450 proteins that depend on it. The spectrum of hormonal and skeletal features seen in affected individuals generally reflects how much residual enzyme activity remains after the variants have exerted their effects.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Cytochrome P450 oxidoreductase deficiency develops when an individual inherits pathogenic variants in both copies of the POR gene, substantially impairing enzyme function and consequently disrupting the activity of steroid-producing cytochrome P450 proteins. The condition affects sexual development before birth and can present with skeletal malformations when enzyme disruption is severe. Clinical features reflect disrupted steroid hormone production during development. The condition follows autosomal recessive inheritance, meaning affected individuals carry variants on both gene copies.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic POR variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
POR holds green (high-evidence) status on multiple NHS Genomic Medicine Service panels, reflecting strong gene-disease validity. These include the Arthrogryposis panel (R83), Differences in sex development panel (R146), Fetal anomalies panel (R21), Likely inborn error of metabolism panel (R98), Rare syndromic craniosynostosis or isolated multisuture synostosis panel (R100), Skeletal dysplasia panel (R104), and the Undiagnosed metabolic disorders panel. This broad representation across NHS testing pathways underscores POR's clinical utility in diagnosing hormone-related developmental anomalies and metabolic conditions within the UK healthcare system.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
How does POR influence medication response?
Cytochrome P450 oxidoreductase supports hepatic cytochrome P450 enzymes that metabolise many common drugs. Natural variation in POR may alter how efficiently these enzymes clear medications, potentially affecting drug efficacy and adverse reaction risk, though the clinical significance of most common polymorphisms is still being defined.
What is cytochrome P450 oxidoreductase deficiency?
Cytochrome P450 oxidoreductase deficiency is a rare autosomal recessive disorder caused by biallelic POR variants. It disrupts steroid hormone synthesis, leading to hormonal changes that can affect the development of the reproductive system, skeleton, and other parts of the body, with severity depending on residual enzyme activity.
Why is POR on multiple NHS testing panels?
POR appears on several NHS Genomic Medicine Service panels because pathogenic variants cause a spectrum of features including skeletal dysplasia, craniosynostosis, differences in sex development, and metabolic abnormalities. Its inclusion across arthrogryposis, foetal anomalies, and metabolic disorder panels reflects the gene's broad clinical relevance in diagnosing hormone-related developmental conditions.