Created at the VGL Lethal White Overo (LWO)

Quick Summary

Lethal white overo (LWO) is a genetic disorder that results from two copies of the version of the gene causing the frame overo coat color pattern.

Frame Overo horse
Horse with frame overo pattern

Click here for Price and Turnaround Time

Phenotype: Lethal white overo (LWO) is a genetic disorder that results from two copies of the version of the gene causing the frame overo coat color pattern. Foals with the condition are characterized by a completely white coat as well as intestinal tract abnormalities that result in death soon after birth.  The frame overo pattern itself is characterized by a white spotting pattern in which pigment is said to "frame" the horse. 

Mode of Inheritance: Incomplete dominance

Alleles: N = Normal or non-white, O = Overo

Breeds appropriate for testing: Paint Horse, Quarter Horse, Miniature Horse, Thoroughbred

Explanation of Results:

  • Horses with N/N genotype will not have the overo pattern and cannot transmit this lethal white overo variant to their offspring.
  • Horses with N/O genotype will have the overo pattern. They may transmit this lethal white overo variant to their offspring. Matings with N/N horses will result in a 50% chance of producing an overo foal. Matings with N/O horses will result in a 25% chance of producing a lethal white foal.
  • Horses with O/O genotype will have LWO, a condition incompatible with life.

Turnaround Time
At least 10 business days; may be delayed beyond 10 business days if sample requires additional testing, or a new sample is requested.

$40 one test per animal
+ $15 each additional coat color/type test on the same animal

Sample Collection

Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).

1. Grab about 10 hairs at the base.

2. Wrap the hairs around your finger and give it a quick pull.

3. Check the ends to make sure the pulled hairs have roots.

4. Repeat the process until you have collected about 20-30 hairs with intact roots.

5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only. 

6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.

Hairs with roots

7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.


Additional Details

Lethal white overo (LWO) is a homozygous; lethal condition associated with the frame overo white spotting pattern.  The frame overo pattern, like all other white spotting patterns, can vary along a continuum from a horse with very minimal white to a horse with lots of white.  This particular pattern was named after horses from in the middle of the range upwards, that is the pigmented area is said to “frame in the horse” and thus white patterning primary occurs on the abdomen, middle of the neck, and on the face.  

Foals born with the lethal white overo syndrome are affected by intestinal tract abnormalities, leading to colic and eventual death. Horse breeding programs specializing in the frame overo pattern have particular challenges compared with programs for other white patterns such as tobiano. When breeding two horses with the frame pattern, not only is there the possibility of producing a solid dark foal without the overo pattern, but there is also the risk of producing an all-white foal that dies of complications from intestinal tract abnormalities (ileocolonic aganglionosis).

This was the first white spotting pattern to have the genetic cause determined in horses and was discovered at the Veterinary Genetics Laboratory. The mutation responsible for the LWO condition is a two base pair mutation (p.Ile118Lys) in the gene endothelin B receptor gene (EDNRB).  EDNRB  is responsible for the proper development of both pigment cells and nerve cells during embryogenesis. Mutations in this same gene cause a similar disease in humans known as Hirschsprung disease.  Horses with two copies of the mutation (O/O) have the lethal condition as there is no treatment. These foals usually die shortly after birth.   Horses with one copy of mutation (N/O) have a frame overo pattern but do not have any known intestinal complications but  maybe deaf. 

Breeders can test horses for the identified mutation to avoid producing lethal white foals. Some horses with a very minimal white spotting pattern have one copy of the LWO allele (O) and can also therefore produce affected foals.   Matings between two N/O horses will results in a 25% chance of producing an LWO affected foal. Therefore, testing for LWO is important to assist veterinarians to make the correct diagnosis and to assist owners in management and breeding decisions. 


We know of no other mutations that have been associated with lethal white frame overo horses. However, owners requesting the diagnostic test should understand that there is the rare possibility that two NN horses could have a lethal white foal if both the sire and dam carry a mutation at a site other than the one detected by this test.