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Reviewed January 2015

What is the official name of the OPN1MW gene?

The official name of this gene is “opsin 1 (cone pigments), medium-wave-sensitive.”

OPN1MW is the gene's official symbol. The OPN1MW gene is also known by other names, listed below.

Read more about gene names and symbols on the About page.

What is the normal function of the OPN1MW gene?

The OPN1MW gene provides instructions for making a protein that is essential for normal color vision. This protein is found in the retina, which is the light-sensitive tissue at the back of the eye. The retina contains two types of light receptor cells, called rods and cones, that transmit visual signals from the eye to the brain. Rods provide vision in low light. Cones provide vision in bright light, including color vision. There are three types of cones, each containing a specific pigment (a photopigment called an opsin) that is most sensitive to particular wavelengths of light.

The OPN1MW gene provides instructions for making an opsin pigment that is more sensitive to light in the middle of the visible spectrum (yellow/green light). Cones with this pigment are called middle-wavelength-sensitive or M cones. In response to light, the photopigment triggers a series of chemical reactions within an M cone. These reactions ultimately alter the cell's electrical charge, generating a signal that is transmitted to the brain. The brain combines input from all three types of cones to produce normal color vision.

People can have one or more copies of the OPN1MW gene in each cell. All copies of this gene are located in a row on the X chromosome near another opsin pigment gene, OPN1LW. The OPN1LW gene provides instructions for making a photopigment that is more sensitive to light at long wavelengths (in the orange/red part of the visible spectrum). A nearby region of DNA, known as the locus control region (LCR), regulates the activity of the OPN1MW and OPN1LW genes. Only the two opsin pigment genes nearest the LCR, generally the OPN1LW gene and the first copy of the OPN1MW gene, are active in the retina and contribute to color vision.

Does the OPN1MW gene share characteristics with other genes?

The OPN1MW gene belongs to a family of genes called GPCR (G protein-coupled receptors).

A gene family is a group of genes that share important characteristics. Classifying individual genes into families helps researchers describe how genes are related to each other. For more information, see What are gene families? in the Handbook.

How are changes in the OPN1MW gene related to health conditions?

color vision deficiency - caused by mutations in the OPN1MW gene

Several kinds of genetic changes involving the OPN1MW gene cause red-green color vision defects, a form of color vision deficiency that makes it difficult or impossible to distinguish between shades of red, yellow, and green. Most red-green color vision defects result from structural rearrangements involving the OPN1LW and OPN1MW genes. Because these genes are so similar, they occasionally swap genetic material when the genes are being passed from parent to child. This swapping, called recombination, can ultimately delete genetic material from one or both genes or lead to the formation of a hybrid pigment gene that contains part of the OPN1MW gene and part of the OPN1LW gene.

Less commonly, red-green color vision defects can result from an OPN1MW gene mutation that changes a single protein building block (an amino acid) in the middle-wave-sensitive photopigment. This mutation replaces the amino acid cysteine with the amino acid arginine at position 203 (written as Cys203Arg or C203R).

When OPN1MW gene mutations lead to completely nonfunctional M cones, color vision depends entirely on the other two types of cones. The specific type of red-green color vision deficiency that results from a total loss of M cone function is called deuteranopia. A less severe red-green color vision defect called deuteranomaly occurs when a partially functional hybrid pigment gene replaces the normal OPN1MW gene. The photopigments made from these hybrid genes usually have abnormal visual properties that impair red-green color vision.

A rarer form of color vision deficiency, blue cone monochromacy, severely reduces sharpness of vision (visual acuity) and affects the ability to perceive most colors. This condition also includes other vision problems that are not typically found with red-green color vision defects. Blue cone monochromacy occurs when genetic changes prevent the opsin pigments produced from both the OPN1MW and OPN1LW genes from functioning normally. In some cases, the condition is caused by a deletion of the LCR, which normally controls the activity of the OPN1MW and OPN1LW genes. A loss of the LCR prevents the production of pigments from both genes. As a result, people with this condition have only functional cones with short-wavelength-sensitive photopigment (S cones), which leads to reduced visual acuity and poor color vision. The cone abnormalities also underlie the other vision problems in people with blue cone monochromacy.

Where is the OPN1MW gene located?

Cytogenetic Location: Xq28

Molecular Location on the X chromosome: base pairs 154,182,596 to 154,196,861

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBIThis link leads to a site outside Genetics Home Reference.)

The OPN1MW gene is located on the long (q) arm of the X chromosome at position 28.

The OPN1MW gene is located on the long (q) arm of the X chromosome at position 28.

More precisely, the OPN1MW gene is located from base pair 154,182,596 to base pair 154,196,861 on the X chromosome.

See How do geneticists indicate the location of a gene? in the Handbook.

Where can I find additional information about OPN1MW?

You and your healthcare professional may find the following resources about OPN1MW helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the OPN1MW gene or gene products?

  • CBBM
  • GCP
  • green cone photoreceptor pigment
  • green cone pigment
  • green-sensitive opsin
  • middle-wave-sensitive pigment
  • M-pigment
  • OPN1MW1
  • photopigment apoprotein

Where can I find general information about genes?

The Handbook provides basic information about genetics in clear language.

These links provide additional genetics resources that may be useful.

What glossary definitions help with understanding OPN1MW?

amino acid ; arginine ; cell ; chromosome ; cones ; cysteine ; deficiency ; deletion ; DNA ; gene ; locus ; mutation ; photopigment ; photoreceptor ; pigment ; protein ; receptor ; retina ; rods ; spectrum ; tissue ; visual acuity

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.

See also Understanding Medical Terminology.

References (7 links)


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.

Reviewed: January 2015
Published: February 8, 2016