WARMBLOOD fragile foal syndrome (WFFS) is a lethal genetic disease that affects a small but increasing proportion of warmblood foals. A mutation within the LH1 gene causes the problem by affecting cross linking of collagen fibres. There is a recessive pattern of inheritance and any foal who inherits a copy of the detrimental mutation from each of its parents will either suffer a late miscarriage or need to be euthanised at or shortly after birth suffering from extreme skin fragility, muscle hypotonia, joint laxity particularly of the fetlocks, and a range of other problems.

The parents of affected foals carry one normal and one mutated copy of the gene, and are therefore known as carriers though they appear normal. The prevalence of the WFFS mutation is increasing rapidly within the warmblood breeding population, increasing the risk of WFFS foals and raising dilemmas that are difficult for breeders and potentially for top-level competition.