Curly coated rats were written about as early as the 1930s, and there are multiple genes that cause the slightly harsh and bouncy curl that is so prized in rex show rats.

The fur on rex rats varies through their life. Males tend to have curlier fur, but in both sexes they can sometimes go through an odd kitten moult at around 4-5 weeks where they drop lots of fur and area almost bald in patches. Older rex rats often slowly become more and more bald, eventually looking quite moth-eaten.

Genetics: Most commonly Rere (one copy of the rex gene, one copy of the standard coat gene). There are multiple genes that produce curly fur, and several of them are present in the fancy (Cu1, Cu2, Sh), but generally for ease of use the symbol Re is used. There are also recessive rex genes (kk, wvwv, fzfz) which are less common.

NFRS Standard: The coat to be evenly dense and not excessively harsh, with as few guard hairs as possible. Coat to be evenly curled and also to a lesser extent on the belly. Curly vibrissae (whiskers) are normal for Rex. Colour to conform to a recognised colour or pattern variety. Where Silver or ticked rats are rexed, allowances should be made for the lower number of guard hairs present than in normal varieties.