Chicken eggs with blue-green coloured eggshells have been known for centuries. The pigmentation is due to deposition of biliverdin IX and biliverdin zinc chelate in eggshell and underlying membranes during their formation. The deposition of pigment is determined by a dominant autosomal gene, marked with О. According to most recent research, the gene responsible for this mutation is SLCO1B3. In the world, there are some breeds and breed groups of chickens that lay eggs with blue-green eggshells. During the last two decades there has been an increasing interest in non-traditionally coloured eggs in West Europe, North America and Japan. Consequently, some egg-laying chicken strains laying blue-green eggs have been created. All non-purebred and hybrid layers producing blue-green eggs are collectively called Easter Eggers and contribute to the maintenance of biological diversity in domestic chickens.