The Rex Gene

MisterMiceGuy is focusing on breeding mice who have curly/wavy hair. The gene responsible for this trait is called Rex (Re) and is located on the chromosome 11 (Crew & Auerbach 1939, Sundberg and King Jr 1996). The Rex gene is also commonly known Astrex (The Finnish Mouse Club 2020).

A short haired young adult male exhibiting slightly wavy hair due to being heterozygous for the Rex gene (Photo Credit: MisterMiceGuy 2019).

Rex mice were originally described by Crew and Auerbach (1939) when they received some mice from a Mr. Tuck who worked at the Rayleigh Rat and Mouse Farms in Essex, England. The gene was named after Rex Rabbits that had a similar coat texture (Crew & Auerbach 1939).

Today, in the fancy mouse hobby, there are inconsistencies regarding the casual naming of mouse genes and phenotypes (Robbin 2018, The Finnish Mouse Club 2020). Regardless of its casual name, the Rex gene should not be confused with a variety of other similar but distinct genes such as frizzy (Fr), Caracul (Ca), Wellhaarig (we), Waved-1 (wa-1), or Waved-2 (wa-2) (Spacek et al. 2010, Carter 1951). Mice with curly fur of unknown genetic origin are sometimes described as “Rexoid” (Carter 1951).

(A) Newborn mouse with typical closed eyelid (Toonen, Liang, and Sidjanin 2012). This would be the typical eye lid of a mouse with the Rex gene. B) Newborn mouse with open eyelid due to wave-2 gene (Photo Credit: Toonen, Liang, and Sidjanin 2012).

Rex and Caracal are described as being very similar or identical in phenotype but originating from different genes (Crew & Auerbach 1939). Rex and Waved-2 occur on the same chromosome and are loosely linked (Carter 1951). There are some interactions we between Rex and the waved-1 and waved-2 varieties (Carter 1951). A notable difference between Rex and waved-2 is the presence of open eyelids at birth (Toonen, Liang, and Sidjanin 2012).

Non-Rex pups can be identified early due to their straight whiskers (Photo Credit: MisterMiceGuy 2019).

Although once thought to a be a complete dominant autosomal gene (Crew and Auerbach 1939) it has been since discovered that the Rex gene has incomplete dominance (Carter 1951). Heterozygous mice (ie mice with one copy of the Rex gene) will have smoother coats, looser waves, and more widely spread whiskers. Homozygous mice (with two copies of the Rex gene) have rough coats, whiskers that heavily curve inwards towards the mouse and have been described as “walrus” like (Carter 1951).

Baby heterozygote displaying whiskers that turn away from the face at the tip (Photo Credit: MisterMiceGuy 2019).

The difference between the phenotype of heterozygous and homozygous Rex mice has been thoroughly described by Carter (1951). On Day 7 Homozygous mice can be identified by heavily forward curled “walrus-like” whiskers and guard hairs on the neck and rear that curl forward. Heterozygous mice have whiskers and guard hairs that appear longer and straighter and are curled outwards just below the tip of the hair shaft (Carter 1951). As of March 2020 MisterMiceGuy does not have any homozygous mice to use to illustrate the “walrus-like” whiskers. Pictures to come as soon as homozygotes are produced.

Juvenile male heterozygote displaying the loose ripple wave pattern (Photo Credit: MisterMiceGuy 2019).

On Day 11 through 20 homozygous mice have a ripple wave pattern that appears earlier, is tighter, and has a rougher appearance when compared to heterozygotes. Carter (1951) states that homozygotes might have a crest that develops along the midline on the dorsal and ventral surfaces however MisterMiceGuy has noted that this crest also appears in heterozygotes. This may be due to hobbyist having increased the quality of the Rex curls by selective breeding. Regardless the midline crest tends to disappear with age (Carter 1951).

A young adult male heterozygous for the Rex gene. The curled whiskers and coat have straightened considerably with age (Photo Credit: MistermiceGuy 2019).

By 3 weeks the coat tends to lose much of its curl and ripple effect but homozygous individuals will retain more of the curl and will have a rougher appearance than heterozygous individuals (Carter 1951)

The Rex gene is a fun and easy gene to work with due to its semi-dominant nature. This is especially true for new breeders as breeding a homozygous rex to a non-rex will produce all rex pups and breeding a heterozygous rex to a non-rex will produce half rex pups (Carter 1951). For dramatic effect, Rex can also be combined with long haired genes to produce Texels (Robbins 2018).

Young adult male that is homozygous for long hair and heterozygous for the Rex gene (Photo Credit: MisterMiceGuy 2019).


Carter, T. (1951) Wavy-coated mice: Phenotypic interactions and linkage tests between rex and (a) waved-1, (b) waved-2. Journal of Genetics, 50, 268–276.

Crew, F.A.E., Auerbach, C. (1939) Rex: A dominant autosomal monogenic coat texture character in the mouse. Journal of Genetics, 38341.

Robbins, K (2018) Texel Mice (a.k.a. Long Haired Frizzie). American Fancy Rat and Mouse association, retrieved from

Spacek, D., Perez, A., Ferranti, K., Wu, L., Moy, D., King, T. (2010) The mouse frizzy (fr) and rat ‘hairless’ (frCR) mutations are natural variants of protease serine S1 family member 8 (Prss8). Experimental Dermatology, 19, 527-532.

Sundberg, J (1994) Handbook of Mouse Mutations with Skin and Hair Abnormalities: Animal Models and biomedical tools. CRC Press, pp. 407.

Sundberg, J., King Jr., L (1996) Mouse Mutations as Animal Models and Biomedical Tools for Dermatological Research. Journal of Investigative Dermatology, (106)2, 368-376.

The Finnish Mouse Club (2020). Genetics. Retrieved from:

Toonen, J., Liang, L. & Sidjanin, D.J. (2012) Waved with open eyelids 2 (woe2) is a novel spontaneous mouse mutation in the protein phosphatase 1, regulatory (inhibitor) subunit 13 like (Ppp1r13l)gene. BMC Genet 13, 76.


Published by Ryan David Tuttle

PhD Graduate student studying Behavioral Neuroscience, Addiction, Stress, Behavioral Economics, and Individual Differences. Former member Ministerial Servant and Pioneer in a Spanish speaking congregation of Jehovah's Witnesses.

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