## 3 Molecular Characterization of Coat Color Affecting Genes in Rabbits
### A Marker in the Fibroblast Growth Factor 5 (FGF5) Gene is Associated to the Angora Locus  
The _Angora_ locus is due to an autosomal recessive allele (indicated with \"\(l\)\") that determines an abnormal long hair growth with no other modifications on fiber structure or coat composition [105; 106; 107; 108]. Hair growth lasts about 13 weeks in Angora rabbits, whereas it lasts only six weeks in the rabbits that are not homozygous for the mutated \(l\) allele. In mice, mutations disrupting the fibroblast growth factor 5 (_Fgf5_) gene produce an angora-like phenotype [109]. Using a candidate gene approach based on the evidences reported in mice, [PERSON] et al. [110] investigated the rabbit _FGF5_ gene and identified a missense mutation in exon 3 that was in strong (even if not complete) linkage disequilibrium with the _Angora_ allele. This gene, assigned to rabbit chromosome 15, was included in the chromosome region in which the _Angora_ locus was located based on segregation analysis followed by microsatellite markers in rabbit families, further supporting the candidacy of _FGF5_ in this phenotype [111]. Transcriptome analyses based on RNA-seq of skin of long-hair Angora and short-hair Rex rabbits have shown that _FGF5_ gene expression was upregulated in long-haired rabbits [112; 113]. This high _FGF5_ expression level in Angora rabbits is quite surprising considering that FGF5 has a role in inhibiting hair growth and inducing catagen through a regression of hair follicles [114]. This inhibiting role has been also demonstrated in rabbits when CRISPR/Cas9 mediated disruption of the _FGF5_ gene produced a long hair phenotype by prolonging anagen [115]. It could be possible that alternatively spliced _FGF5_ transcripts or other regulatory events might be involved in suppressing FGF5 activities in Angora rabbit skin [113; 116]. Other gene expression studies in the skin of Angora rabbits have been carried out to clarify the regulating mechanisms determining anagen length in this model and the changes in gene expression over the hair cycle [117; 118; 119], but the causative mutation(s) of the Angora phenotype in rabbits has (have) not been identified yet.  
Molecular Characterization of Other Loci Determining Breed and Strain Specific or Un-Specific Phenotypes  
Other phenotypic traits that characterize different rabbit breeds or strains and that have been investigated at the molecular level are related to the body size and type of locomotion. Two of them specifically identify pet rabbits (dwarfism) or a unique strain with a peculiar locomotion behavior (Table 1). On the other hand, among the altered phenotypes, a few have (or had in the past) a quite broad diffusion in different breeds. This is the case of the yellow fat trait caused by the _Yellow fat_ locus that has been characterized at the molecular level (Table 1). A more detailed description of the mutants and related causative variants or associated DNA markers is reported below.  
Several putative loci affecting morphological traits (e.g., ear length, vertebral number) or causing gross abnormalities of the eyes, nervous system, and skeleton as well as of several other tissues and systems have been described in some other lines or strains [30] but have not been analyzed at the molecular level yet.