Involved in the regulation of melanogenesis. The binding of ASP to MC1R precludes alpha-MSH initiated signaling and thus blocks production of cAMP, leading to a down-regulation of eumelanogenesis (brown/black pigment) and thus increasing synthesis of pheomelanin (yellow/red pigment). The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. A polymorphism exists that is responsible for the nonagouti phenotype, characterized by a plain black coat on the back and belly. This is due to a 19-bp deletion resulting in a frameshift at position 36 and a premature stop codon at position 48. Both strain WKAH (agouti) and strain BN (nonagouti) contain a substitution at the splice donor site of intron 3, resulting in a shorter isoform lacking exon 3 which causes reduced expression levels in strain WKAH and almost undetectable levels in strain BN.