The extension or E locus codes for a G protein linked cell surface receptor, found on the surface of melanocytes, known as Melanocortin receptor type I (McR1). Activation of this receptor by alpha-Melanocyte Stimulating Hormone (MSH), leads to a G protein cascade resulting in eumelanin production. ASIP, coded for at the agouti locus antagonises this receptor causing a shift from eumelanin to pheomelanin production. Regulated expression of agouti gives rise to the banded hair shaft seen in the agouti patterned rabbits. In a way the E locus is like a switch, which can be turned on by MSH and turned off by ASIP.

Mutations of the E locus leading to inactivation of this receptor have a similar effect as "turning off" Mc1R (with ASIP), however this is a permanent effect, as such pheomelanin is continually expressed, this gives an orange phenotype.

E locus Alleles

E^s
E
E^j
e


The human varient of Mc1R is responsible producing red hair, it has also been implicated in fair skin and may be involved in melanomas.