ST Locus (Screw Tail, Bulldog and Terrier Type)

Other Names: Robinow-like Syndrome
Affected Genes: DVL2
Inheritance: Autosomal Incomplete Dominant with Incomplete Penetrance and Variable Expressivity
Mutation: chr5:32195044 (canFam3): 1 bp deletion (del G)
Breed(s): American Bulldog, American Bully, American Pit Bull Terrier, American Staffordshire Terrier, Boston Terrier, Bull Terrier, Bulldog, Bullmastiff, French Bulldog, Staffordshire Bull Terrier
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Testing Summary

The ST Locus (Screw Tail) test reliably determines if a dog has one of the following genotypes at the ST locus:


This dog does not carry a copy of ST and has a genotype of N/N which does not result in a screw or kinked tail in the bully breeds. This dog will pass one copy of N to 100% of its offspring.

Interpretation: No kinked or screw tail


This dog carries one copy of ST which may result in a kinked tail. However, the degree to which a dog’s tail is affected cannot be determined. This dog will pass one copy of ST to 50% of its offspring and one copy of N to 50% of its offspring.

Interpretation: May have limited kinked tail


This dog carries two copies of ST which results in a kinked or screw tail in bully breed dogs. Although the DVL2 Mutation consistently affects the presentation of the tail, dogs with this mutation may have other vertebral abnormalities. This dog will pass one copy of ST to 100% of its offspring.

Interpretation: Kinked or screw tail

Detailed Summary

The last section of the vertebral column, or tail, can have as many as 23 caudal vertebrae. Dogs with variation in the number, or development of the caudal vertebrae, may present with a shortened and kinked tail. This is often referred to as a “screw tail”. This is a distinctive morphological trait of the Bulldog, French Bulldog and Boston Terrier and is associated with having two copies of the DVL2 Mutation. Having just one copy of the DVL2 mutation may result in a partially kinked tail but the degree to which the tail is kinked may be variable. Although this mutation is responsible for a defining trait of these bully breeds, other mutations are likely responsible for the other distinguishing features of these dogs’ conformation. Other breeds related to these bully breeds that also exhibit caudal vertebral abnormalities that present as a kinked or screw tail should also have one or two copies of the DVL2 mutation.

Testing Tips

Genetic testing of the DVL2 gene will reliably determine whether or not a dog is a genetic Carrier of the variant that produces a kinked or screw tail. Breeding two dogs with screw tails (ST/ST) should reliably conserve this trait in the offspring.

NOTE: Mutation of DVL2 is also associated with other caudal vertebral malformations, brachycephalic phenotypes including brachycephalic obstructive airway syndrome, and congenital heart defects. These features are characteristic of Robinow syndrome in humans and have led some scientists to suggest that mutation of DVL2 leads to a Robinow-like syndrome in dogs. However, it is also clear that the genetic mutations responsible for the distinguishing features of the bully breeds are not all known.

There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.


  • Mansour TA, Lucot K, Konopelski SE, Dickinson PJ, Sturges BK, Vernau KL, Choi S, Stern JA, Thomasy SM, Döring S, Verstraete FJM, Johnson EG, York D, Rebhun RB, Ho HH, Brown CT, Bannasch DL. Whole genome variant association across 100 dogs identifies a frame shift mutation in DISHEVELLED 2 which contributes to Robinow-like syndrome in Bulldogs and related screw tail dog breeds. PLoS Genet. 2018 Dec 6;14(12):e1007850. [PubMed: 30521570]
  • Niskanen JE, Reunanen V, Salonen M, Bannasch D, Lappalainen AK, Lohi H, Hytönen MK. Canine DVL2 variant contributes to brachycephalic phenotype and caudal vertebral anomalies. Hum Genet. 2021 Nov;140(11):1535-1545. doi: 10.1007/s00439-021-02261-8. Epub 2021 Feb 18. [PubMed: 33599851]